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/__tests__/run.test.js

@@ -190,6 +190,15 @@ describe("/api/run", () => {
     tmux.__setExecImpl(() => "tmux 3.4");
     const { body } = await fetchJson("/api/run/tmux");
     assert.equal(body.available, true);
+    // The version and this platform's upgrade advice (null when new enough).
+    assert.equal(body.version, "3.4");
+    assert.equal(body.upgrade, tmux.upgradeAdvice("3.4"));
+  });
+
+  it("GET /tmux says to upgrade a version older than the platform asks for", async () => {
+    tmux.__setExecImpl(() => "tmux 2.9");
+    const { body } = await fetchJson("/api/run/tmux");
+    assert.match(body.upgrade, /older than/);
   });
 
   it("GET /cwds still returns suggested directories (unchanged behavior)", async () => {