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.

client/src/components/lanes/__tests__/LaneGitBar.test.tsx

@@ -16,6 +16,8 @@ import { LaneGitBar } from "../LaneGitBar";
 import { api } from "../../../lib/api";
 import type { LaneGitFacts } from "../../../lib/types";
 
+// A merge asks through the themed confirm (its host lives in App): yes.
+vi.mock("../../../lib/confirm", () => ({ confirmDialog: vi.fn(async () => true) }));
 vi.mock("../../../lib/api", () => ({
   gitTarget: (lane: number, session?: string | null) => (session ? { lane, session } : lane),
   api: { lanes: { gitAction: vi.fn(), gitBranches: vi.fn() } },
@@ -371,7 +373,6 @@ describe("git login (issue #45)", () => {
 
 describe("merge (issue #45)", () => {
   it("merges the picked branch into the current one after a confirm", async () => {
-    const confirm = vi.spyOn(window, "confirm").mockReturnValue(true);
     const user = userEvent.setup();
     render(<LaneGitBar laneId={3} git={facts({ dirty: 0, untracked: 0 })} onChanged={vi.fn()} />);
     await user.click(screen.getByRole("button", { name: /Branch/ }));
@@ -380,11 +381,9 @@ describe("merge (issue #45)", () => {
     await waitFor(() =>
       expect(api.lanes.gitAction).toHaveBeenCalledWith(3, "merge", { branch: "development" })
     );
-    confirm.mockRestore();
   });
 
   it("offers to merge the upstream when the branch has diverged", async () => {
-    vi.spyOn(window, "confirm").mockReturnValue(true);
     const user = userEvent.setup();
     render(<LaneGitBar laneId={3} git={facts({ dirty: 0, untracked: 0 })} onChanged={vi.fn()} />);
     await user.click(screen.getByRole("button", { name: "Merge origin/feat/thing" }));