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__/SessionResumeButton.test.tsx

@@ -3,8 +3,8 @@
  * @description The transcript page's explicit Resume and Fork: it shows only for a
  * Claude session inside a workspace, a live session offers its pane instead,
  * and a resume asks first (it re-reads the whole transcript), then starts in
- * the first free pane slot and opens that pane — or refuses when all four are
- * busy; a Fork asks too and starts a new session id beside a live original.
+ * the free pane slot the server picks (`slot: "auto"`) and opens that pane —
+ * or says so when all four are busy; a Fork asks too and starts a new session id beside a live original.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
@@ -81,7 +81,7 @@ describe("SessionResumeButton", () => {
     expect(screen.getByRole("dialog")).toHaveTextContent("whole transcript");
     await user.click(screen.getByRole("button", { name: "Resume session" }));
     expect(api.lanes.action).toHaveBeenCalledWith(3, "start", {
-      slot: 1,
+      slot: "auto",
       resumeSessionId: "s-1",
     });
     await waitFor(() =>
@@ -97,7 +97,7 @@ describe("SessionResumeButton", () => {
     await user.click(await screen.findByRole("button", { name: "Resume" }));
     await user.click(screen.getByRole("button", { name: "Resume session" }));
     expect(api.lanes.action).toHaveBeenCalledWith(4, "start", {
-      slot: 0,
+      slot: "auto",
       resumeSessionId: "s-2",
     });
   });
@@ -135,7 +135,7 @@ describe("SessionResumeButton", () => {
     expect(screen.getByRole("dialog")).toHaveTextContent("NEW session");
     await user.click(screen.getByRole("button", { name: "Fork session" }));
     expect(api.lanes.action).toHaveBeenCalledWith(3, "start", {
-      slot: 1,
+      slot: "auto",
       resumeSessionId: "s-1",
       forkSession: true,
     });
@@ -147,7 +147,7 @@ describe("SessionResumeButton", () => {
     await user.click(await screen.findByRole("button", { name: "Resume" }));
     await user.click(screen.getByRole("button", { name: "Resume session" }));
     expect(api.lanes.action).toHaveBeenCalledWith(3, "start", {
-      slot: 1,
+      slot: "auto",
       resumeSessionId: "s-wt",
     });
   });
@@ -178,7 +178,12 @@ describe("SessionResumeButton", () => {
       (slot) => ({ id: `r${slot}`, laneId: 3, slot, status: "running" }) as RunHandle
     );
     await user.click(screen.getByRole("button", { name: "Resume session" }));
-    expect(api.lanes.action).toHaveBeenCalledWith(3, "start", { slot: 2, resumeSessionId: "s-1" });
+    // The server picks the free tab under its lock; the client only re-reads
+    // to open a pane that took this session meanwhile.
+    expect(api.lanes.action).toHaveBeenCalledWith(3, "start", {
+      slot: "auto",
+      resumeSessionId: "s-1",
+    });
   });
 
   it("starts once on a double click while the panes are being re-read", async () => {
@@ -216,16 +221,31 @@ describe("SessionResumeButton", () => {
     expect(screen.getByTestId("where").textContent).toBe("/workspace?lane=3&run=agenthub-lane-3-2");
   });
 
-  it("refuses when all four panes are busy", async () => {
-    runs = [0, 1, 2, 3].map(
-      (slot) => ({ id: `r${slot}`, laneId: 3, slot, status: "running" }) as RunHandle
+  it("says so when the server finds all four panes busy", async () => {
+    vi.mocked(api.lanes.action).mockRejectedValueOnce(
+      Object.assign(new Error("every tab of this workspace is busy"), { code: "ERUNLIVE" })
     );
     const user = userEvent.setup();
     renderButton({ id: "s-1", cwd: "/r", tool: "claude" });
     await user.click(await screen.findByRole("button", { name: "Resume" }));
     await user.click(screen.getByRole("button", { name: "Resume session" }));
-    expect(screen.getByRole("alert")).toHaveTextContent("All four panes");
-    expect(api.lanes.action).not.toHaveBeenCalled();
+    expect(await screen.findByRole("alert")).toHaveTextContent("All four panes");
+  });
+
+  it("opens the pane the server names when the session is already live there", async () => {
+    vi.mocked(api.lanes.action).mockRejectedValueOnce(
+      // A nested workspace's pane holds it: opened in THAT workspace.
+      Object.assign(new Error("open"), { code: "ESESSIONLIVE", runId: "agenthub-lane-4-3" })
+    );
+    const user = userEvent.setup();
+    renderButton({ id: "s-1", cwd: "/r", tool: "claude" });
+    await user.click(await screen.findByRole("button", { name: "Resume" }));
+    await user.click(screen.getByRole("button", { name: "Resume session" }));
+    await waitFor(() =>
+      expect(screen.getByTestId("where").textContent).toBe(
+        "/workspace?lane=4&run=agenthub-lane-4-3"
+      )
+    );
   });
 
   it("shows nothing outside a workspace or for a non-Claude session", async () => {