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/remote-sessions.md

@@ -265,11 +265,12 @@ and it follows the same rule as the launch path: every ssh command line comes fr
 |---------|--------------|
 | `GET /api/sessions/:id/file-raw` | Streamed over `ssh` (`cat`, or `tail -c +N \| head -c L` for a `Range`); the same 200/206/416 contract as a local file, so `<video>`/`<audio>` seeking works |
 | `GET /api/sessions/:id/file-content` | `cat` into memory, capped by the existing text limit; `edit=1` answers `400` (see below) and `editable` is always `false` |
+| `PUT /api/sessions/:id/file-content` | `400` before any path is looked at: the guard sits AHEAD of the local path validation, because with a same-named directory on the Codeman host (an `sshfs` mount) the write would otherwise land on the local twin |
 | `GET /api/sessions/:id/file-preview` | Non-office files redirect to `file-raw` (which works remotely); docx/pptx answer `400` |
 | `GET /api/sessions/:id/file-thumbnail` | `400` for remote files |
 | `POST /api/sessions/:id/attachments` | Registers an absolute path that lives on the **remote** host (a clicked link pointing outside the case directory) by probing it there |
 | `GET /api/sessions/:id/attachments/:attachmentId/raw` | Streams the registered remote file over ssh, same 200/206/416 contract; `preview` (office) and `thumbnail` answer `400` |
-| `GET /api/sessions/:id/attachments/:attachmentId`, `GET …/attachments` (history) | Size/mtime/existence resolved over ssh, so a remote entry is not reported `missing` |
+| `GET /api/sessions/:id/attachments/:attachmentId`, `GET …/attachments` (history) | Size/mtime/existence resolved over ssh, so a remote entry is not reported `missing`; the history list resolves EVERY entry in one batched probe, never one connection per entry |
 
 ⚠️ The attachment route is the one a clicked path takes when it is **outside** the case
 directory (a remote `/tmp` scratchpad capture, a screenshot elsewhere in the home dir):
@@ -284,13 +285,27 @@ by the transport:
    connection is opened.
 3. ONE ssh round trip that returns `realpath` **and** `stat` for the path **and** the
    workspace root (`remoteProbePaths`). Resolving the root remotely is what keeps the
-   boundary honest for a symlinked `remotePath`; the probe uses `readlink -f` when
-   available and a POSIX `cd`/`pwd -P` fallback otherwise.
+   boundary honest for a symlinked `remotePath`. The probe uses `readlink -f` when
+   available; on a host without it (macOS before 12.3) a POSIX fallback canonicalizes
+   the directory chain with `cd -P`/`pwd -P` and then follows the LAST component with
+   plain `readlink` for a bounded number of hops. ⚠️ **The fallback fails closed**: a
+   path it cannot fully resolve (a loop, a `readlink` failure, the hop cap) is reported
+   as unresolvable and answers 404, never as its own unresolved string. An earlier
+   version resolved only the directory chain, so `ws/notes.txt -> ~/.ssh/id_rsa` passed
+   containment under the link's own path while `cat` followed it to the key.
+   Records come back NUL-separated and index-keyed (`<index>|kind|size|mtime|realPath`,
+   after a leading NUL that fences off any login banner), so a filename containing a
+   newline cannot shift the alignment.
 4. Containment of the remote realpath against the remote root. The sensitive-path
    blocklist then applies on whichever routes already apply it locally (`/api/download`,
    attachment registration, edit mode — where resolving symlinks first is what makes it
    meaningful); the remote branch neither drops a guard the local path has nor invents a
-   stricter one.
+   stricter one. One entry of that blocklist is host-bound by construction: the three
+   home-anchored members (`~/.claude.json`, `~/.claude/settings.json`,
+   `~/.claude/settings.local.json`) are compared against the **Codeman host's** home
+   directory, so they do not match a remote home at a different path. Everything else in
+   the list is depth-anchored (`/.ssh/`, `/.aws/credentials`, `/.claude/.credentials.json`,
+   `/etc/shadow`, ...) and applies to a remote path unchanged.
 5. Size cap (`CODEMAN_MAX_DOWNLOAD_BYTES`) applied to the **remote** size, before the
    body is requested.
 
@@ -298,7 +313,20 @@ The path arrives from the browser (`?path=`) and is interpolated as a single
 `shellescape`-quoted token, in a command that is itself shellescaped into the ssh
 line; `BatchMode=yes` means a host needing a passphrase fails fast instead of hanging.
 A failed connection is reported as **502** with the remote reason — never a 404, which
-used to make an unreachable host look like a typo in the agent's output.
+used to make an unreachable host look like a typo in the agent's output. The reason is
+the first stderr line, the timeout, or the exit code; never Node's `Command failed: …`
+message, which would carry the identity-file path and the probe script into the body.
+
+**Connections are bounded.** Every probe and buffered read runs through a small global
+semaphore (`src/remote-ssh-limiter.ts`, default 4, `CODEMAN_MAX_REMOTE_FILE_SSH`), the
+attachment-history list resolves its whole history in one batched probe instead of one
+handshake per entry, and probes are chunked at 40 paths per round trip. Terminal output
+in a remote session is written on the remote host, so a prompt-injected agent printing
+hundreds of `codeman://attach` links used to make the server fork one `ssh` per link,
+each holding a 20 s probe timeout, and a 100-entry history re-listed on every
+`attachment:detected` event tripped OpenSSH's default `MaxStartups 10:30:100`. Streams
+(`file-raw`, by-id `raw`) are not counted: one is held per browser request for the life
+of a playback, and each is gated behind a counted probe anyway.
 
 ⚠️ **There is deliberately NO local fallback.** A remote case reads the remote bytes or
 fails, even when a file with the same absolute name exists on the Codeman host — which