fix(terminal): psmux wheel scrolling, per-platform tmux upgrade advice, Windows-safe server tests

- Server tests on Windows no longer reach the user's real psmux: psmux
  ignores TMUX_TMPDIR (and 3.3.x PSMUX_DATA_DIR), so quit.test.js's Quit and
  the runner's kill-server ended every live lane pane — the pre-commit hook
  included. scripts/test-server.js now takes tmux off the suites' PATH there.
- Wheel scrolling on Windows: psmux never turns mouse tracking on for its
  client, so xterm.js never reported the wheel and a pane could not scroll.
  A Windows attach is told {type:"wheel"} and the browser sends the SGR wheel
  report itself; psmux scrolls or forwards it to the pane's app.
- tmux.upgradeAdvice(): the oldest tmux AgentHub asks for and how each
  platform upgrades (psmux 3.4.0 -> winget upgrade marlocarlo.psmux; tmux 3.7
  -> brew upgrade tmux / build the release on Linux). Used by the doctor,
  GET /api/run/tmux ({available, version, upgrade}), `agenthub doctor`, and a
  sticky toast on the Workspace page. Advice only: nothing installs tmux.

Committed with --no-verify (user's call): the pre-commit hook runs the full
backend suite, which has ~378 pre-existing failures on Windows (S3/Linux-only
tests: sudo, /proc, systemd, members). Every test touching this change passes:
pty-attach, tmux, run, plugin-doctor-tmux, TerminalView, Workspace, and the
client tests related to the changed files; tsc and the header audit are clean.

server/lib/pty-attach.js

@@ -136,6 +136,11 @@ function attach(ws, runId, { cols, rows, platform = process.platform }) {
   group.viewers.add(me);
   if (lead) tell(me, { type: "size", cols: lead.cols, rows: lead.rows });
   else group.driver = me;
+  // psmux never turns mouse tracking on for its client (not even for a pane
+  // app that asked), so xterm.js would never report the wheel and the pane
+  // could not scroll. psmux does act on a wheel report it receives: the
+  // browser sends one itself (TerminalView).
+  if (platform === "win32") tell(me, { type: "wheel" });
 
   // Backpressure: a slow viewer (a remote browser on a thin link) must not
   // queue a chatty pane's output in this process without bound. Past 1 MB

server/lib/tmux.js

@@ -695,6 +695,40 @@ function clientVersion() {
   }
 }
 
+// The oldest tmux AgentHub asks for, and how each platform upgrades it.
+// Windows' is psmux: before 3.4.0 a dashboard terminal stalled until a
+// refresh, and fixes for frame push, writes and synchronized output landed in
+// 3.3.6–3.4.0. Elsewhere 3.7, the first with pane scrollbars (enableMouse).
+const UPGRADES = {
+  win32: {
+    min: [3, 4, 0],
+    name: "psmux",
+    how: "close every AgentHub terminal pane, then run: winget upgrade marlocarlo.psmux",
+  },
+  darwin: { min: [3, 7, 0], name: "tmux", how: "brew upgrade tmux" },
+  linux: {
+    min: [3, 7, 0],
+    name: "tmux",
+    how:
+      "distros ship older ones (Ubuntu 24.04 has 3.4), so build the release from " +
+      "https://github.com/tmux/tmux/releases (needs libevent-dev libncurses-dev bison: " +
+      "./configure && make && sudo make install)",
+  },
+};
+
+/** What to tell the user when `version` is older than their platform asks
+ *  for; null when it is new enough or cannot be compared. Advice only —
+ *  AgentHub never installs or restarts tmux. */
+function upgradeAdvice(version = clientVersion(), platform = process.platform) {
+  const want = UPGRADES[platform];
+  const m = /(\d+)\.(\d+)(?:\.(\d+))?/.exec(version || "");
+  if (!want || !m) return null;
+  const have = [m[1], m[2], m[3] || 0].map(Number);
+  const i = have.findIndex((n, k) => n !== want.min[k]);
+  if (i === -1 || have[i] > want.min[i]) return null;
+  return `${want.name} ${version} is older than ${want.min.join(".")} — ${want.how}`;
+}
+
 /** The RUNNING server's version, null when none runs. A server started before
  *  an upgrade keeps its old version until every tmux session has closed. */
 function serverVersion() {
@@ -725,6 +759,7 @@ module.exports = {
   isTmuxAvailable,
   clientVersion,
   serverVersion,
+  upgradeAdvice,
   serverPid,
   memberAttach,
   __setExecImpl,