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/pages/Workspace.tsx

@@ -665,6 +665,25 @@ export function Workspace() {
   const { local } = usePermissions();
   const [searchParams, setSearchParams] = useSearchParams();
 
+  // The server's tmux is older than this platform needs (psmux < 3.4.0 on
+  // Windows, tmux < 3.7 elsewhere): say how to upgrade it, until it is.
+  // Sticky (an error toast) so the command can be copied. An older server or
+  // no answer says nothing.
+  useEffect(() => {
+    let live = true;
+    (async () => {
+      try {
+        const { upgrade } = await api.run.tmuxAvailable();
+        if (live && upgrade) toast.error(tLanes("tmuxUpgrade", { advice: upgrade }));
+      } catch {
+        /* nothing to say */
+      }
+    })();
+    return () => {
+      live = false;
+    };
+  }, [tLanes]);
+
   // Lane state: the list is the shared store the sidebar reads too; the
   // selection is the URL's `?lane=` (the sidebar sets it, so does a
   // notification click), falling back to the last one shown, then the first.

client/src/pages/__tests__/Workspace.test.tsx

@@ -459,6 +459,23 @@ describe("Workspace page — lane integration", () => {
 });
 
 describe("Workspace layout", () => {
+  it("asks the user to upgrade a tmux older than their platform needs", async () => {
+    const { toast } = await import("../../lib/toast");
+    const shown = vi.spyOn(toast, "error");
+    const run = api.run as unknown as Record<string, unknown>;
+    run.tmuxAvailable = vi.fn().mockResolvedValue({
+      available: true,
+      version: "3.3.2",
+      upgrade: "psmux 3.3.2 is older than 3.4.0 — winget upgrade marlocarlo.psmux",
+    });
+    onTestFinished(() => {
+      delete run.tmuxAvailable;
+      shown.mockRestore();
+    });
+    await renderWorkspace();
+    expect(shown).toHaveBeenCalledWith(expect.stringContaining("winget upgrade marlocarlo.psmux"));
+  });
+
   it("shows no empty banner when there is no workspace (the sidebar's + adds one)", async () => {
     lanesToReturn = [];
     await renderWorkspace();