fix(files): fail closed on remote symlinks, guard PUT for remote cases, bound ssh fan-out

Follow-up to #421 (remote-case file reads over ssh), addressing the review.

Symlink escape on a host without `readlink -f` (blocker). The probe's
portable fallback canonicalized only the directory chain and returned the
final component unresolved, so on macOS < 12.3 `ws/notes.txt -> ~/.ssh/id_rsa`
came back as `.../ws/notes.txt` (with the target's size), passed every
containment and blocklist check that runs on `realPath`, and `cat` followed
the link. The fallback now walks the directory chain with `cd -P`/`pwd -P`
and follows the LAST component with plain `readlink` for a bounded number of
hops, and anything it cannot fully resolve (a loop, a readlink failure, the
hop cap) is reported with an `x` marker that parses as null, i.e. 404. It
never returns the unresolved string. Measured on a real /bin/sh with
`readlink -f` shadowed: the pre-fix script reports `/ws/notes.txt`, the fixed
one `/secret/id_rsa`; both branches (native and fallback) now agree.

`PUT /api/sessions/:id/file-content` never had the remote guard the PR
described. It sits ahead of `validateSessionFilePath`, which resolves against
the LOCAL filesystem, because with a same-named directory on the Codeman host
(an sshfs mount of the remote tree, the documented stop-gap) the write landed
on the local twin while the viewer believed it edited the remote file.

ssh fan-out is bounded. `src/remote-ssh-limiter.ts` is a
document-conversion-limiter-shaped semaphore (default 4, env
`CODEMAN_MAX_REMOTE_FILE_SSH`) around every probe and buffered read; the
attachment-history list resolves its whole history in ONE batched probe
(`probeRemoteAttachmentHistory`, threaded into
`registerExternalAttachment({remoteProbes})` so the guards run unchanged)
instead of one handshake per entry; and probes chunk at 40 paths because the
whole script is one argv string. Terminal output in a remote session is
written on the remote host, so a prompt-injected agent printing hundreds of
`codeman://attach` links forked one ssh per link, each holding a 20 s
timeout, and a 100-entry history re-listed on every attachment:detected
tripped OpenSSH's default MaxStartups. Streams are deliberately not counted
(one per browser request, held for a whole playback, and gated behind a
counted probe anyway).

Smaller items from the same review: probe records are NUL-terminated and
index-keyed after a leading NUL (a newline in a filename can no longer shift
the alignment, and the banner is fenced off without last-N-lines guessing);
size comes from `stat -c %s || stat -f %z`; the three IO functions refuse
under VITEST instead of opening a connection; an unreachable host now reads
as unknown (missing: false) for detected AND external history entries, where
external used to fold its 502 into missing; a client that aborted during the
guard probe has its body's ssh child reaped (`reply.raw.destroyed` is checked
before the close listener is attached); `describeExecError` never returns
Node's `Command failed: <ssh line>` message, which carried the identity path
and the probe script into a 502 body; and the docs note that
`isSensitivePath`'s three home-anchored entries resolve against the Codeman
host's home, not the remote one.

Tests: the probe script runs on a real /bin/sh with a `readlink` shim that
rejects `-f` (the escape, a relative chain through a symlinked directory, a
loop, a newline filename, banner chatter that itself looks like a record),
the limiter's cap and FIFO order, and route tests for the PUT guard (local
twin untouched, no connection), the single batched history probe, the
unreachable-host alignment and the aborted-client reap. All four route tests
fail against the pre-fix file-routes.ts.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/architecture-invariants.md

@@ -54,7 +54,7 @@ Model is NOT a session field: it is a composition entry in the profile's config
 
 ### Remote SSH cases
 
-**Remote SSH cases** (COD-94/#145): cases can point at a **remote host** (`~/.codeman/remote-hosts.json` + `remote-cases.json` via `src/remote-hosts.ts`; CRUD under `/api/cases` — cases route file). A remote session launches a LOCAL tmux pane running `ssh <host>` that creates a durable REMOTE tmux session on a **dedicated socket** `-L codeman-remote` with name `codeman-ssh-<id>` — deliberately failing the remote Codeman's `SAFE_MUX_NAME_PATTERN` so a Codeman instance on the target host never adopts it; no `-g` global tmux options are set remotely. `remotePath`/`identityFile` are schema-guarded against shell injection (backticks/`$` rejected — same approach as `extraSshOptions`); remote tmux availability is probed via `checkRemoteTmuxAvailable()` in quick-start (ssh args carry `-o ConnectTimeout=10`). Remote claude defaults to an idempotent `claude --session-id <id> || claude --resume <id>` pair under a login shell, so a respawn or reattach continues the SAME conversation rather than starting a fresh one (remote omp gets the same treatment via `--continue`; ⚠️ because the claude arm is an `a || b` pair under `-c`, that pane's PID is the login shell, not the agent); per-host `commands.*` override. Session kill best-effort kills the remote tmux too. `SessionState.remote`/`MuxSession.remote` round-trip through recovery (`restoreMuxSessions` passes `remote` back into the Session constructor). ⚠️ Run flows must route remote cases through `POST /api/quick-start` (which resolves the remote case and skips LOCAL CLI availability gates) — `POST /api/sessions` stat-validates `workingDir` locally and has no `caseName`. `envOverrides`/`effort`/`modelOverride`/`codexConfig`/`geminiConfig` are rejected for remote quick-starts (not silently dropped). UI: Create Case modal → Remote tab. Tests: `test/remote-hosts.test.ts`, `test/remote-ssh-options.test.ts`. ⚠️ **Reading a file in a remote case goes over ssh too** (#415): `src/remote-files.ts` is the single remote-READ layer (`buildRemoteFileCommand` = `buildSshConnectionArgs` + one shellescaped remote command; `remoteProbePaths` returns remote realpath + stat; `remoteCreateReadStream` streams a `Range` via `tail -c +N | head -c L` and its `close()` must be wired to the response's `close` or the ssh child outlives an aborted download). The guard order matches the local path exactly (`validateSessionFilePathLexical` → remote realpath of BOTH file and workspace root → containment → sensitive-path → size cap on the REMOTE size), a request path arrives from the browser and is only ever interpolated as a `shellescape`d token, and an unreachable host answers **502**, never a 404. This covers the ATTACHMENT routes too, which is the half a clicked path needs when the file is OUTSIDE the case directory (`_isExternalPreviewPath` sends it to `POST …/attachments`): registration, by-id `raw`, metadata and the history list all resolve over ssh (`registerExternalAttachment({remote})`, `resolveServableRemoteAttachment`), and what decides the host is the SESSION, never the path string — the same absolute path means a different file on each host. Deliberately NOT supported over ssh: writes (`edit=1`/`PUT` answer 400, `editable` is always false), office previews/thumbnails, the file tree/picker, `tail-file`. Tests: `test/remote-files.test.ts`, `test/routes/file-routes-remote.test.ts`.
+**Remote SSH cases** (COD-94/#145): cases can point at a **remote host** (`~/.codeman/remote-hosts.json` + `remote-cases.json` via `src/remote-hosts.ts`; CRUD under `/api/cases` — cases route file). A remote session launches a LOCAL tmux pane running `ssh <host>` that creates a durable REMOTE tmux session on a **dedicated socket** `-L codeman-remote` with name `codeman-ssh-<id>` — deliberately failing the remote Codeman's `SAFE_MUX_NAME_PATTERN` so a Codeman instance on the target host never adopts it; no `-g` global tmux options are set remotely. `remotePath`/`identityFile` are schema-guarded against shell injection (backticks/`$` rejected — same approach as `extraSshOptions`); remote tmux availability is probed via `checkRemoteTmuxAvailable()` in quick-start (ssh args carry `-o ConnectTimeout=10`). Remote claude defaults to an idempotent `claude --session-id <id> || claude --resume <id>` pair under a login shell, so a respawn or reattach continues the SAME conversation rather than starting a fresh one (remote omp gets the same treatment via `--continue`; ⚠️ because the claude arm is an `a || b` pair under `-c`, that pane's PID is the login shell, not the agent); per-host `commands.*` override. Session kill best-effort kills the remote tmux too. `SessionState.remote`/`MuxSession.remote` round-trip through recovery (`restoreMuxSessions` passes `remote` back into the Session constructor). ⚠️ Run flows must route remote cases through `POST /api/quick-start` (which resolves the remote case and skips LOCAL CLI availability gates) — `POST /api/sessions` stat-validates `workingDir` locally and has no `caseName`. `envOverrides`/`effort`/`modelOverride`/`codexConfig`/`geminiConfig` are rejected for remote quick-starts (not silently dropped). UI: Create Case modal → Remote tab. Tests: `test/remote-hosts.test.ts`, `test/remote-ssh-options.test.ts`. ⚠️ **Reading a file in a remote case goes over ssh too** (#415): `src/remote-files.ts` is the single remote-READ layer (`buildRemoteFileCommand` = `buildSshConnectionArgs` + one shellescaped remote command; `remoteProbePaths` returns remote realpath + stat; `remoteCreateReadStream` streams a `Range` via `tail -c +N | head -c L` and its `close()` must be wired to the response's `close` or the ssh child outlives an aborted download). The guard order matches the local path exactly (`validateSessionFilePathLexical` → remote realpath of BOTH file and workspace root → containment → sensitive-path → size cap on the REMOTE size), a request path arrives from the browser and is only ever interpolated as a `shellescape`d token, and an unreachable host answers **502**, never a 404. ⚠️ The probe's symlink resolution FAILS CLOSED: `readlink -f` where it exists, otherwise a `cd -P`/`pwd -P` directory walk plus a bounded plain-`readlink` loop over the last component, and anything it cannot fully resolve is reported unresolvable (404), never as the unresolved string — the first version resolved the directory chain only, so on a host without `readlink -f` a `ws/notes.txt -> ~/.ssh/id_rsa` link passed containment under its own path while `cat` served the key. Records are NUL-separated and index-keyed so a newline in a filename cannot shift the mapping. ⚠️ ssh children are BOUNDED: probes and buffered reads go through `src/remote-ssh-limiter.ts` (a `document-conversion-limiter`-shaped semaphore, default 4), the attachment-history list probes its whole history in ONE batched call (`probeRemoteAttachmentHistory`, threaded into `registerExternalAttachment({remoteProbes})`), and probes chunk at 40 paths — a prompt-injected agent printing `codeman://attach` links in a remote session used to fork one `ssh` per link. `describeExecError` never returns Node's `Command failed: <ssh line>` message (identity path + probe script in a 502 body). The `PUT /file-content` guard sits AHEAD of `validateSessionFilePath`, which resolves LOCALLY, or a same-named local directory (an sshfs mount) takes the write. Under `VITEST` the three IO functions refuse rather than connect. This covers the ATTACHMENT routes too, which is the half a clicked path needs when the file is OUTSIDE the case directory (`_isExternalPreviewPath` sends it to `POST …/attachments`): registration, by-id `raw`, metadata and the history list all resolve over ssh (`registerExternalAttachment({remote})`, `resolveServableRemoteAttachment`), and what decides the host is the SESSION, never the path string — the same absolute path means a different file on each host. Deliberately NOT supported over ssh: writes (`edit=1`/`PUT` answer 400, `editable` is always false), office previews/thumbnails, the file tree/picker, `tail-file`. Tests: `test/remote-files.test.ts`, `test/routes/file-routes-remote.test.ts`.
 
 ### Docker cases