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.

client/src/components/run/__tests__/TerminalView.test.tsx

@@ -231,6 +231,24 @@ describe("TerminalView", () => {
     expect(wheelHandlerMock.mock.calls[0]![0]!(new Event("wheel"))).toBe(false);
   });
 
+  it("sends the wheel itself once the server says its tmux never tracks the mouse", () => {
+    // psmux (Windows) never turns mouse tracking on for its client, so xterm
+    // never reports the wheel and the pane could not be scrolled at all.
+    termState.modes.mouseTrackingMode = "none";
+    termState.buffer.active.type = "alternate";
+    render(<TerminalView runId="agenthub-lane-1" wsBaseUrl="ws://localhost:4820" />);
+    const ws = MockWebSocket.instances[MockWebSocket.instances.length - 1]!;
+    ws.onmessage?.({ data: '{"type":"wheel"}' });
+    const wheel = wheelHandlerMock.mock.calls[wheelHandlerMock.mock.calls.length - 1]![0]!;
+    expect(wheel(new WheelEvent("wheel", { deltaY: -100 }))).toBe(false);
+    expect(wheel(new WheelEvent("wheel", { deltaY: 100 }))).toBe(false);
+    expect(ws.sent).toEqual(["\x1b[<64;1;1M", "\x1b[<65;1;1M"]);
+    // Tracked again (a tmux that does report): xterm sends it, never twice.
+    termState.modes.mouseTrackingMode = "any";
+    expect(wheel(new WheelEvent("wheel", { deltaY: -100 }))).toBe(true);
+    expect(ws.sent).toHaveLength(2);
+  });
+
   it("keeps Shift+Tab in the terminal instead of letting focus escape", () => {
     render(<TerminalView runId="agenthub-lane-1" wsBaseUrl="ws://localhost:4820" />);
     const handler = keyHandlerMock.mock.calls[0]![0]! as (e: KeyboardEvent) => boolean;