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  (POST /api/users/:id/tmux-restart) or every idle outdated one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/lib/linux-accounts.js

@@ -46,6 +46,13 @@ function serverConf() {
   return confCache.value;
 }
 
+let TMUX_MEMBER = "/opt/agenthub/libexec/tmux-member";
+/** Test seam: where tmux-member is looked for. */
+function __setTmuxMember(p) {
+  TMUX_MEMBER = p;
+  confCache = { at: 0, path: null, value: null };
+}
+
 function readConf(file) {
   let text;
   try {
@@ -56,16 +63,28 @@ function readConf(file) {
   const get = (k) => new RegExp(`^${k}=(.*)$`, "m").exec(text)?.[1]?.trim() || "";
   const usersRoot = get("USERS_ROOT");
   if (!path.isAbsolute(usersRoot)) return null;
+  // A member's tmux runs through tmux-member once install-system put it there
+  // (and in sudoers): it picks their RUNNING server's own binary, so a tmux
+  // upgrade never mixes a new client with an old server. Before that, TMUX_BIN.
+  let tmuxBin = get("TMUX_BIN");
+  try {
+    fs.accessSync(TMUX_MEMBER, fs.constants.X_OK);
+    if (path.isAbsolute(tmuxBin)) tmuxBin = TMUX_MEMBER;
+  } catch {
+    /* not installed yet */
+  }
   return {
     hostUser: get("HOST_USER"),
     hostUid: Number(get("HOST_UID")),
     usersRoot,
     port: Number(get("PORT")) || null,
+    // The installed tmux (a new member server starts on it).
+    tmuxInstalled: path.isAbsolute(get("TMUX_BIN")) ? get("TMUX_BIN") : null,
     // The absolute paths sudoers lists (sudo would resolve a bare name on its
     // own secure_path, which may not be them).
     bins: Object.fromEntries(
       [
-        ["tmux", get("TMUX_BIN")],
+        ["tmux", tmuxBin],
         ["git", get("GIT_BIN")],
         ["gh", get("GH_BIN")],
       ].filter(([, v]) => path.isAbsolute(v))
@@ -154,6 +173,7 @@ function __resetConfCache() {
 
 module.exports = {
   __resetConfCache,
+  __setTmuxMember,
   confPath,
   LINUX_NAME_RE,
   serverConf,