feat(workspace): no double resume, server-picked tabs, terminal a11y

- A resume of a session a live pane already holds is refused (409
  ESESSIONLIVE with error.run_id) and the client opens that pane, pasting
  any typed message there; catalog tools refuse resumeSessionId
  (EUNSUPPORTED) instead of silently starting fresh.
- POST /api/lanes/:id/start takes slot: "auto": the server picks the free
  tab under the lane lock (components/lanes/startSession.ts on the client,
  kept out of WorkspaceHome so the sidebar does not pull react-markdown
  into the main bundle).
- Unsaved whole-file edits are guarded on every exit (Back, another file,
  tab switch, close, lane/session change, beforeunload).
- Closing an AI tab asks first; hiding a pane has its own icon. Tabs and
  pane headers show the session name, a state dot, rename by double-click.
- Terminal: Ctrl+M toggles Tab-moves-focus (no keyboard trap), Cmd+F /
  Ctrl+Shift+F find bar, clickable links (noopener), a write-only OSC 52
  clipboard, a Reconnecting overlay, debounced resizes, optional screen
  reader mode. Alt+1-4 / Alt+\ / Alt+W focus, split and hide panes.
- Changes panel: draggable splitter, wrapping git bar, history paging,
  disarming discard/undo/abort, inline diff comments pasted only into a
  visible pane; drag MIME guard on sidebar rows; IME-safe Enter; Show
  more in the sidebar and Recents; idle dots readable with sr-only text.
- Adds @xterm/addon-clipboard, @xterm/addon-search, @xterm/addon-web-links.

server/__tests__/pty-run.test.js

@@ -596,6 +596,21 @@ describe("pty-run", () => {
       assert.deepEqual(paneArgv(last), [at("agy"), "--continue", "--dangerously-skip-permissions"]);
     });
 
+    // A catalog tool has no "resume THIS session" argv; starting it fresh under
+    // a resume request would read as a resume that silently lost the history.
+    it("refuses resumeSessionId for a catalog tool instead of starting fresh", () => {
+      assert.throws(
+        () =>
+          pty.spawnRun({
+            laneId: 15,
+            cwd: "/tmp/repo",
+            tool: "antigravity",
+            resumeSessionId: "abc12345",
+          }),
+        { code: "EUNSUPPORTED" }
+      );
+    });
+
     it("refuses a flag-shaped model id for any tool, before tmux is asked", () => {
       for (const tool of ["claude", "codex", "antigravity"]) {
         assert.throws(