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  (POST /api/users/:id/tmux-restart) or every idle outdated one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-panes.test.js

@@ -240,9 +240,20 @@ describe("run-as: a path under the users root belongs to that user", () => {
       process.env.AGENTHUB_SERVER_CONF,
       `HOST_USER=h\nHOST_UID=1\nUSERS_ROOT=${TMP}/users\nGIT_BIN=/usr/bin/git\nTMUX_BIN=/opt/x/tmux\n`
     );
-    linux.__resetConfCache();
+    linux.__setTmuxMember(path.join(TMP, "no-tmux-member"));
     assert.equal(runAs.sudoArgv(ACCT, "git", ["status"])[1][3], "/usr/bin/git");
     assert.equal(runAs.sudoArgv(ACCT, "tmux", [])[1][3], "/opt/x/tmux");
+    // Once install-system put tmux-member there, a member's tmux goes through
+    // it (their running server's own binary); the installed one is still known.
+    const wrapper = path.join(TMP, "tmux-member");
+    fs.writeFileSync(wrapper, "#!/bin/sh\n", { mode: 0o755 });
+    linux.__setTmuxMember(wrapper);
+    try {
+      assert.equal(runAs.sudoArgv(ACCT, "tmux", [])[1][3], wrapper);
+      assert.equal(linux.serverConf().tmuxInstalled, "/opt/x/tmux");
+    } finally {
+      linux.__setTmuxMember(path.join(TMP, "no-tmux-member"));
+    }
     assert.equal(
       runAs.sudoArgv(ACCT, "/opt/agenthub/libexec/write-as", [])[1][3],
       "/opt/agenthub/libexec/write-as"