feat(server-s3): a member's tmux follows tmux upgrades — tmux-member + Users → Members' tmux

A member's tmux server keeps the version it started with until their last
pane closes, but the dashboard asked it with TMUX_BIN, the newest tmux: a
3.8 client on a 3.6 server spun it at 100% CPU (three members needed a
manual kill on 2026-10-15).

- /opt/agenthub/libexec/tmux-member (install-system, in sudoers): runs a
  member's tmux call with the binary of THEIR running server
  (/proc/<pid>/exe, found from the socket in /proc/net/unix), else
  TMUX_BIN. It runs as the member and execs only a process they own.
  sudoArgv uses it once installed (linux-accounts readConf), TMUX_BIN
  until then.
- Users → Members' tmux (GET /api/users/tmux): each member server's version
  beside the installed one, sessions, agents working. An admin restarts one
  (POST /api/users/:id/tmux-restart) or every idle outdated one
  (POST /api/users/tmux-restart-idle), with their password again, audited
  per member; a server that will not stop answers 409 ETMUXSTUCK naming
  its pid for root.
- server-s3-system.test.js: `delete` is a provision action since Delete
  user, so the "unknown action" case now uses `purge`.

Operators: run `sudo agenthub server install-system` once.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

client/src/i18n/locales/en/users.json

@@ -204,5 +204,35 @@
     "body": "This deletes {{name}} for good: their login, {{count}} workspace(s), and their Linux account with the whole home folder (/srv/agenthub/users/{{user}}) — every file and all source code in it. It cannot be undone. Session history and the audit log stay.",
     "confirm": "Delete for good",
     "needsPassword": "Enter your own password below first."
+  },
+  "tmux": {
+    "title": "Members' tmux",
+    "description": "Each member's own tmux server keeps the version it started with until its last pane closes. Installed: tmux {{version}}.",
+    "refresh": "Refresh",
+    "notMatched": "Run `sudo agenthub server install-system` once: until then an older member server is asked with the installed tmux, which can leave it stuck at 100% CPU.",
+    "empty": "No member has a tmux server running.",
+    "user": "User",
+    "version": "tmux",
+    "sessions": "Sessions",
+    "agents": "Agents",
+    "stuck": "not answering",
+    "outdated": "older than installed",
+    "since": "since {{at}}",
+    "working_one": "{{count}} working",
+    "working_other": "{{count}} working",
+    "idle": "idle",
+    "restart": "Restart {{name}}'s tmux",
+    "restartIdle_one": "Restart {{count}} idle member",
+    "restartIdle_other": "Restart {{count}} idle members",
+    "restartIdleTitle": "Restart idle members' tmux",
+    "restartHint": "Every pane of {{name}} closes ({{count}} sessions). Their Claude sessions resume from the workspace, and the next start runs the installed tmux.",
+    "restartHintBusy": "An agent of {{name}} is working right now and will be stopped. Every pane of theirs closes ({{count}} sessions); their Claude sessions resume from the workspace.",
+    "restartIdleHint": "Restarts the tmux of every member on an older version with no agent working: {{names}}. Their panes close; their Claude sessions resume from the workspace.",
+    "restartButton": "Restart",
+    "done": "{{name}}'s tmux restarted",
+    "doneIdle": "Restarted: {{names}}",
+    "errors": {
+      "ETMUXSTUCK": "The server did not stop. Run the `sudo kill -9` command shown, as root, on the server."
+    }
   }
 }