fix: v2.7.3 — Update now restarts through our own service unit, member panes stay live on a dev server, a start refuses a missing folder

- self-restart: a server in our unit's cgroup (/agenthub.service, system
  or user) restarts via `systemctl restart --no-block` (sudo -n for the
  system unit), our launchd job by exiting into KeepAlive. The detached
  restart-helper replacement held the port while Restart=always started
  the unit every 3 s (v2.7.2 on the S3 box: 240+ restarts, MainPID 0).
- run-as.mayHaveTmux: only ENOENT means "no tmux". A dev server in server
  mode without CAP_DAC_READ_SEARCH got EACCES on a member's 0700
  .agenthub/tmux, dropped the member from the list and ended their runs
  within seconds — the real cause of `401 not a live pane's token`.
- tmux: a member's list is re-asked every 5 s, not 1 s (one sudo + three
  auth.log lines each).
- POST /api/lanes/:id/start: 409 ENOCWD for a folder that no longer
  exists (the pane started in the account's home otherwise).
- Settings: Remote sources only for staff (a developer got a 403).
- scripts/test-server.js drops every inherited AGENTHUB_* (pane env).
Tests: self-restart.test.js, server-s3-panes (mayHaveTmux), lanes-api
(ENOCWD); touched suites 771 + client pages 191, tsc -b.

docs/API.md

@@ -357,6 +357,8 @@ Error `409 ESESSIONLIVE` is returned when `resumeSessionId` (or `resume`) names
 
 Error `400 EBADPROMPT` is returned for a `prompt` (or `POST /api/run`'s `initialPrompt`) over 128 KB: no CLI can take one argument that long (Linux's `MAX_ARG_STRLEN`, issue #68) — attach the text as a file instead. A prompt over 4 KB never rides in tmux's argv (one tmux message holds 16 KB, `command too long`): outside Windows it is written to `<data dir>/run-prompts/<run id>.txt` (0600) and the pane's own `sh` reads it back as the CLI's last argument and deletes the file. An `after-start` tool's paste still goes through `tmux set-buffer` and stays under that 16 KB.
 
+Error `409 ENOCWD` is returned when the workspace's folder no longer exists (or is not a directory) — the pane would otherwise start in the account's home under this workspace's name. A folder this server cannot read (`EACCES`) is not refused.
+
 Error `409 ERUNLIVE` is returned if the workspace already has a live run; stop it first with the `stop` action. The same code and status come back when the workspace's tmux pane is already running an agent while a `prompt` or `resumeSessionId` was sent — that argv cannot be typed over a live agent, so the request is refused instead of answered 200 with nothing done. A start that asks for neither still adopts the existing pane silently.
 
 #### Preflight a destructive workspace action