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/__tests__/WorkspaceTree.test.tsx

@@ -44,6 +44,11 @@ const session = (id: string, name: string) =>
 
 let lanes: Lane[] = [];
 
+// Archiving a live session asks through the themed confirm (App's host): yes.
+vi.mock("../../lib/confirm", () => ({
+  confirmDialog: vi.fn(async () => true),
+  confirmDiscard: vi.fn(async () => true),
+}));
 vi.mock("../../lib/api", async (importOriginal) => {
   const actual = await importOriginal<Record<string, unknown>>();
   return {
@@ -65,6 +70,10 @@ vi.mock("../../lib/api", async (importOriginal) => {
           sessions: [session("s-live", "Live one"), session("s-old", "Old one")],
         })),
         patch: vi.fn(async () => ({ session: {} })),
+        // A tab's session with no row yet (a bare tab): 404.
+        get: vi.fn(async () => {
+          throw Object.assign(new Error("not found"), { status: 404 });
+        }),
       },
       run: {
         list: vi.fn(async () => ({
@@ -483,6 +492,6 @@ describe("workspaceDot", () => {
     expect(workspaceDot(lane(1, "/a", { status: "error" }))).toBe("bg-status-danger");
     expect(workspaceDot(lane(1, "/a", { status: "running" }))).toContain("animate-pulse-slow");
     expect(workspaceDot(lane(1, "/a", { liveness: "active" }))).toBe("bg-status-success");
-    expect(workspaceDot(lane(1, "/a"))).toBe("bg-surface-4");
+    expect(workspaceDot(lane(1, "/a"))).toBe("border border-fg-muted");
   });
 });