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/pages/__tests__/Workspace.test.tsx

@@ -55,6 +55,8 @@ let nextRunId = 1;
 // tests are about the session view it hides, so it is a stub here.
 vi.mock("../../components/lanes/WorkspaceHome", () => ({
   WorkspaceHome: () => <div data-testid="workspace-home" />,
+}));
+vi.mock("../../components/lanes/startSession", () => ({
   paneRunId: (id: number, slot: number) =>
     slot ? `agenthub-lane-${id}-${slot}` : `agenthub-lane-${id}`,
   freeSlot: (laneId: number, runs: Array<{ laneId: number; slot?: number; status: string }>) => {
@@ -63,6 +65,25 @@ vi.mock("../../components/lanes/WorkspaceHome", () => ({
     );
     return [0, 1, 2, 3].find((s) => !busy.has(s)) ?? null;
   },
+  // The server picks the slot (`slot: "auto"`); stand in for it with the first
+  // one the mocked run list leaves free.
+  startInFreeTab: async (laneId: number, body: Record<string, unknown>) => {
+    const { api } = await import("../../lib/api");
+    const { items } = await api.run.list();
+    const busy = new Set(
+      (items as Array<{ laneId: number; slot?: number; status: string }>)
+        .filter((r) => r.laneId === laneId && r.status === "running")
+        .map((r) => r.slot ?? 0)
+    );
+    const slot = [0, 1, 2, 3].find((x) => !busy.has(x)) ?? 0;
+    await api.lanes.action(laneId, "start", { ...body, slot: "auto" });
+    return slot ? `agenthub-lane-${laneId}-${slot}` : `agenthub-lane-${laneId}`;
+  },
+}));
+// Closing an AI tab asks first (the themed confirm needs App's host): yes.
+vi.mock("../../lib/confirm", () => ({
+  confirmDialog: vi.fn(async () => true),
+  confirmDiscard: vi.fn(async () => true),
 }));
 // An ended session's replay (its own suite): a stub that names its session.
 vi.mock("../../components/lanes/SessionReplay", () => ({
@@ -243,6 +264,9 @@ vi.mock("../../lib/api", async (importOriginal) => {
       },
       sessions: {
         list: vi.fn().mockResolvedValue({ sessions: [], total: 0, limit: 50, offset: 0 }),
+        get: vi.fn().mockImplementation(async (id: string) => ({
+          session: { id, cwd: "/workspace/a", tool: "claude" },
+        })),
         transcript: vi.fn().mockResolvedValue({ messages: [] }),
       },
     },
@@ -594,7 +618,7 @@ describe("console tabs", () => {
     const user = userEvent.setup();
     await user.click(screen.getByTestId("console-tab-new"));
     await user.click(within(screen.getByTestId("console-tab-menu")).getByText("New terminal"));
-    expect(api.lanes.action).toHaveBeenCalledWith(1, "start", { slot: 1, shell: true });
+    expect(api.lanes.action).toHaveBeenCalledWith(1, "start", { slot: "auto", shell: true });
   });
 
   it("disables + once all four slots are taken", async () => {
@@ -782,12 +806,15 @@ describe("split panes", () => {
 
     await right(screen.getByTestId("pane-header-1"));
     await user.click(screen.getByTestId("pane-menu-split-bottom"));
-    expect(api.lanes.action).toHaveBeenCalledWith(1, "start", { slot: 3, shell: true });
+    expect(api.lanes.action).toHaveBeenCalledWith(1, "start", { slot: "auto", shell: true });
+    // The new terminal (the slot the server picked, 3) opened under pane 1.
+    await settle();
+    expect(shownPanes()).toEqual(["pane-0", "pane-1", "pane-3"]);
 
     await right(screen.getByTestId("pane-header-1"));
-    await user.click(within(screen.getByTestId("pane-menu-1")).getByText("Close pane"));
+    await user.click(within(screen.getByTestId("pane-menu-1")).getByText("Hide pane"));
     await settle();
-    expect(shownPanes()).toEqual(["pane-0"]);
+    expect(shownPanes()).toEqual(["pane-0", "pane-3"]);
     expect(api.run.kill).not.toHaveBeenCalled();
 
     await right(screen.getByTestId("console-tab-2"));
@@ -804,7 +831,7 @@ describe("split panes", () => {
     const user = userEvent.setup();
     await user.click(screen.getByTestId("pane-split-bottom-1"));
     await user.click(screen.getByTestId("pane-split-new-bottom-1"));
-    expect(api.lanes.action).toHaveBeenCalledWith(1, "start", { slot: 2, shell: true });
+    expect(api.lanes.action).toHaveBeenCalledWith(1, "start", { slot: "auto", shell: true });
   });
 
   it("a split menu starts an AI tool, bare, in the first free slot", async () => {
@@ -815,7 +842,7 @@ describe("split panes", () => {
     const user = userEvent.setup();
     await user.click(screen.getByTestId("pane-split-right-1"));
     await user.click(screen.getByTestId("pane-split-new-right-1-claude"));
-    expect(api.lanes.action).toHaveBeenCalledWith(1, "start", { slot: 2, prompt: "" });
+    expect(api.lanes.action).toHaveBeenCalledWith(1, "start", { slot: "auto", prompt: "" });
   });
 
   it("with one pane, the split menus sit beside Tools and split it", async () => {