fix(files): serve remote-case attachments, the path a click takes outside the case

A clicked path that points OUTSIDE the case directory goes through the attachment
routes (the frontend's `_isExternalPreviewPath` sends every absolute path not under
`workingDir` to `POST /attachments`), and those had the same local-`fs` assumption
as file-raw: `realpathSync`/`fs.stat` on a path that only exists on the remote host,
so the file never opened — the case the #415 report was actually about.

- `registerExternalAttachment()` accepts `remote` and resolves through
  `remoteProbePaths` (canonical path, size/mtime, kind, plus the workspace root for
  the confinement check). Everything around it — blocklist, extension allowlist,
  workspace confinement, registry/dedupe — is now shared by both branches, so the
  remote path cannot drift from the local one.
- The by-id routes (`raw`, `preview`, `thumbnail`), the metadata poll and the
  attachment history list resolve over ssh too. `raw` streams with the same
  Range contract as file-raw; `preview` (office) and `thumbnail` answer 400 for a
  remote record; an unreachable host answers 502, a vanished file 404.
- Which host a record is read from follows the SESSION, never the path string: the
  same absolute path is a different file on each host, and a remote session never
  falls back to a local file with that name.
- Codex generated artifacts keep force-workspace confinement for a remote case: the
  well-known artifact directories are anchored at THIS host's home, so only a file
  inside the remote workspace is trusted.

Still local-only by design: writes, office conversion, thumbnails, the file
tree/picker and tail-file.

CLAUDE.md

@@ -213,7 +213,7 @@ Codeman is a Claude Code session manager with web interface and autonomous Ralph
 
 **Cron (`CronJob`s)**: saved, named jobs on a recurring schedule (`once`/`interval`/`daily`/`weekly`) with per-job run history. ⚠️ **Distinct from the legacy `ScheduledRun`** (`/api/scheduled`, a run-now duration-bounded loop); the two never interact and keep separate `Scheduled*` / `Cron*` names. `CronService` **reuses the existing session layer** rather than rebuilding tmux logic. Next-run math is pure and unit-tested in `cron-time.ts` (server-local timezone). The schedule is advanced BEFORE launch so a slow launch cannot re-trigger. → [architecture-invariants#cron-jobs](docs/architecture-invariants.md#cron-jobs), `docs/cron-discovery.md`
 
-**Remote sessions + remote SSH cases**: a case can point at a remote host. The agent runs inside a durable remote `tmux -L codeman-remote` (session name `codeman-ssh-<id>`, deliberately failing the remote Codeman's `SAFE_MUX_NAME_PATTERN` so an instance on the target host never adopts it), fronted by a LOCAL tmux pane running `ssh`. Attached (`owned:false`) sessions **detach, never kill** on tab close; owned ones propagate `kill-session`. A bounded-backoff watcher auto-reconnects dropped sessions (`remoteAutoReconnect`, default ON). ⚠️ **It revives ONLY when the durable remote tmux session is verifiably still alive** (`remoteTmuxSessionAlive()`, a `has-session` probe over ssh, #355): a clean agent exit (Ctrl-C, Ctrl-D, `exit`) tears that session down, and `isPaneDead()` cannot tell it from a transport drop, so the watcher used to relaunch a FRESH agent after every clean exit (claude only looked fine because its `|| --resume` fallback masked it). An unreachable host answers `undefined`, which also means do not revive. ⚠️ `has-session` prints NOTHING on success, so the probe is classified by EXIT STATUS (`classifyRemoteAliveExit`: 0 alive, ssh's 255 or a timeout unknown, anything else gone); reading stdout classified every live session as gone and silently disabled transport-drop reconnects. The answer is cached per session and forgotten whenever the pane is seen alive again, or a stale `true` from one transport drop would revive the next clean exit. ⚠️ **File reads in a remote case are the second ssh surface** (#415, `src/remote-files.ts`): they go through `buildSshConnectionArgs()` as well, a browser-supplied path is only ever a `shellescape`d token, an unreachable host answers 502 (never 404), the size cap uses the REMOTE size, and no remote file is ever copied onto the server's disk — which is why writes, office previews and thumbnails are deliberately unsupported over ssh. ⚠️ **Command-injection surface: every ssh command line must flow through `buildSshConnectionArgs()`**, which `shellescape`s every user field. Never hand-build an ssh line elsewhere. ⚠️ Run flows must route remote cases through `POST /api/quick-start`, not `POST /api/sessions` (which stat-validates `workingDir` locally and has no `caseName`). → [architecture-invariants#remote-sessions-over-ssh](docs/architecture-invariants.md#remote-sessions-over-ssh), [#remote-ssh-cases](docs/architecture-invariants.md#remote-ssh-cases), `docs/remote-sessions.md`
+**Remote sessions + remote SSH cases**: a case can point at a remote host. The agent runs inside a durable remote `tmux -L codeman-remote` (session name `codeman-ssh-<id>`, deliberately failing the remote Codeman's `SAFE_MUX_NAME_PATTERN` so an instance on the target host never adopts it), fronted by a LOCAL tmux pane running `ssh`. Attached (`owned:false`) sessions **detach, never kill** on tab close; owned ones propagate `kill-session`. A bounded-backoff watcher auto-reconnects dropped sessions (`remoteAutoReconnect`, default ON). ⚠️ **It revives ONLY when the durable remote tmux session is verifiably still alive** (`remoteTmuxSessionAlive()`, a `has-session` probe over ssh, #355): a clean agent exit (Ctrl-C, Ctrl-D, `exit`) tears that session down, and `isPaneDead()` cannot tell it from a transport drop, so the watcher used to relaunch a FRESH agent after every clean exit (claude only looked fine because its `|| --resume` fallback masked it). An unreachable host answers `undefined`, which also means do not revive. ⚠️ `has-session` prints NOTHING on success, so the probe is classified by EXIT STATUS (`classifyRemoteAliveExit`: 0 alive, ssh's 255 or a timeout unknown, anything else gone); reading stdout classified every live session as gone and silently disabled transport-drop reconnects. The answer is cached per session and forgotten whenever the pane is seen alive again, or a stale `true` from one transport drop would revive the next clean exit. ⚠️ **File reads in a remote case are the second ssh surface** (#415, `src/remote-files.ts`): they go through `buildSshConnectionArgs()` as well, a browser-supplied path is only ever a `shellescape`d token, an unreachable host answers 502 (never 404), the size cap uses the REMOTE size, and no remote file is ever copied onto the server's disk — which is why writes, office previews and thumbnails are deliberately unsupported over ssh. The ATTACHMENT routes (a clicked path outside the case dir) go through the same layer, and which host a record is read from follows the SESSION, never the path string. ⚠️ **Command-injection surface: every ssh command line must flow through `buildSshConnectionArgs()`**, which `shellescape`s every user field. Never hand-build an ssh line elsewhere. ⚠️ Run flows must route remote cases through `POST /api/quick-start`, not `POST /api/sessions` (which stat-validates `workingDir` locally and has no `caseName`). → [architecture-invariants#remote-sessions-over-ssh](docs/architecture-invariants.md#remote-sessions-over-ssh), [#remote-ssh-cases](docs/architecture-invariants.md#remote-ssh-cases), `docs/remote-sessions.md`
 
 **Docker cases**: a case can point at a **container**, with any of the CLI run modes running inside it. Like remote-SSH this is a **LOCATION OVERLAY on cases, never a `SessionMode` of its own**. Exactly one long-lived container **per case**, shared by all its sessions, so killing a session kills only that session's in-container tmux and **never** `docker stop` while siblings remain. The workspace is a real host dir bind-mounted at the **same absolute path**, which is what keeps file-routes/watchers on real host bytes and makes the in-container transcript projHash match the host. Credentials are **seeded** (RO mount, copied into the container once) rather than shared RW, so in-container CLIs never write refreshed tokens back to the host, and bind mounts are excluded from `docker commit` so exports stay secret-free. **NEVER a create-time `-e` for secrets, NEVER `--privileged`, NEVER the docker socket.** Config drift is detected via a label hash and a drifted launch is REFUSED rather than silently launched with stale config. ⚠️ A case may instead **ADOPT** a container the user already runs (`DockerCase.owned === false`, mirror of remote-SSH's `owned:false`): Codeman only `exec`s into it and never creates, starts, stops, restarts or removes it, so a missing or stopped container FAILS CLOSED with an actionable message instead of being fixed. Absent = owned, so existing cases are byte-identical. The guarantee is enforced at four independent layers because it cannot be observed by using the feature: `buildDockerStopCommand`/`buildDockerRemoveCommand` throw during pure STRING CONSTRUCTION, `removeDockerContainer` refuses again, drift reports "none" (an adopted container carries no `codeman.confighash` label, so a real comparison would 409 the launch forever), and the boot reaper skips it. ⚠️ Two lifecycle touches the original design missed and that are easy to re-introduce: the full-image export `docker commit`s the container (refused for an adopted case) and the workspace export `docker pause`s it first (skipped — it freezes the owner's processes for the length of the tar). ⚠️ `owned` is applied AFTER `dockerConfigHash`, which takes an explicit field list, or every pre-existing case would trip the drift gate at once. ⚠️ Run modes for a container case come from the CONTAINER (`availableModes`, live-probed): gating the run menu on HOST CLIs (#201) is right for local sessions and wrong here, since a host with no `claude` may run a container that ships one. ⚠️ **A failed probe means opposite things per ownership** — for an ADOPTED case it is a fault worth reporting, for an OWNED one it is the NORMAL state before the first session (the launch chain creates the container), so treating it as a fault hid every agent mode on every freshly linked Docker case behind "start it yourself first". That is why `CaseInfo.docker.owned` is on the wire. ⚠️ Claude is launched WITHOUT `--dangerously-skip-permissions` when the container's exec user is root (Claude Code refuses the flag as root and the refusal is visible only inside the container); which flag to drop is a per-CLI fact, so it is the registry's `overlays.docker.rootCommand`, never a branch. ⚠️ Adoption is **admin-only in multi-user mode**, unlike `docker-link`: linking creates OUR container, whose one bind mount `isWorkingDirAllowed` has already confined, while an adopted container's mounts belong to its owner and one mounting `/` hands the adopter the host. The same reasoning admin-gates the container listing and the in-container directory browser; the preflight instead admits a non-admin for a container already linked to a case they own, because the run menu probes it for every docker case. ⚠️ On the loopback-only prod bind a container cannot reach 127.0.0.1, so in-container hooks need `CODEMAN_DOCKER_BRIDGE_HOOKS=1`; otherwise idle detection falls back to output-based. → [architecture-invariants#docker-cases](docs/architecture-invariants.md#docker-cases), `docs/docker-cases.md` (user guide), `docs/docker-cases-plan.md` (design)