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.

bin/agenthub.js

@@ -1284,12 +1284,14 @@ async function cmdDoctor() {
   // warning, not a failure: it costs the run console and `lanes shell`, nothing
   // else.
   try {
-    const { available } = await get("/api/run/tmux");
+    const { available, upgrade } = await get("/api/run/tmux");
     console.log(
-      available
-        ? `${c.green("✔")}  tmux  available to the server`
-        : `${c.yellow("!")}  tmux  not on the server's PATH — Start Run and \`agenthub workspace shell\` will fail. ` +
+      !available
+        ? `${c.yellow("!")}  tmux  not on the server's PATH — Start Run and \`agenthub workspace shell\` will fail. ` +
             `Install it (sudo apt install tmux / brew install tmux), then restart the server`
+        : upgrade
+          ? `${c.yellow("!")}  tmux  ${upgrade}`
+          : `${c.green("✔")}  tmux  available to the server`
     );
   } catch {
     console.log(`${c.dim("·")}  tmux  not reported by this server`);