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