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.