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  (POST /api/users/:id/tmux-restart) or every idle outdated one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>

server/__tests__/server-s3-system.test.js

@@ -187,6 +187,7 @@ describe("Linux accounts", () => {
       hostUid: 4242,
       usersRoot: "/srv/agenthub/users",
       port: 4820,
+      tmuxInstalled: null,
       bins: {},
     });
     assert.deepEqual(linux.accountFor("mia"), {
@@ -199,7 +200,8 @@ describe("Linux accounts", () => {
     for (const bad of ["Mia", "1mia", "mi.a", "-x", "a".repeat(30), "root;id", "../x"]) {
       assert.throws(() => linux.provision("create", bad), { code: "EBADUSER" }, bad);
     }
-    assert.throws(() => linux.provision("delete", "mia"), /unknown provision action/);
+    // `delete` is an action now (Delete user); anything else is still refused.
+    assert.throws(() => linux.provision("purge", "mia"), /unknown provision action/);
     assert.deepEqual(argvs, []);
   });