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__/tmux.test.js

@@ -326,4 +326,21 @@ describe("tmux wrapper", () => {
     });
     assert.equal(tmux.isTmuxAvailable(), false);
   });
+  it("upgradeAdvice asks each platform for its own minimum, with its own command", () => {
+    // Windows: psmux before 3.4.0 (the WinGet `tmux`).
+    assert.match(
+      tmux.upgradeAdvice("3.3.2", "win32"),
+      /psmux 3\.3\.2 .*3\.4\.0.*winget upgrade marlocarlo\.psmux/
+    );
+    assert.equal(tmux.upgradeAdvice("3.4.0", "win32"), null);
+    assert.equal(tmux.upgradeAdvice("3.10.1", "win32"), null);
+    // macOS / Linux: tmux before 3.7 (pane scrollbars).
+    assert.match(tmux.upgradeAdvice("3.4", "darwin"), /brew upgrade tmux/);
+    assert.match(tmux.upgradeAdvice("3.4", "linux"), /github\.com\/tmux\/tmux\/releases/);
+    assert.equal(tmux.upgradeAdvice("3.7c", "linux"), null);
+    assert.equal(tmux.upgradeAdvice("next-3.8", "darwin"), null);
+    // Nothing to compare: no advice rather than a wrong one.
+    assert.equal(tmux.upgradeAdvice(null, "linux"), null);
+    assert.equal(tmux.upgradeAdvice("3.3.2", "freebsd"), null);
+  });
 });