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.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "agenthub",
-  "version": "2.7.2",
+  "version": "2.7.3",
   "bin": {
     "agenthub": "bin/agenthub.js"
   },