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__/WorkspaceHome.test.tsx

@@ -24,13 +24,24 @@ vi.mock("../../run/RunSetup", () => ({
   PromptEditor: ({
     value,
     onChange,
+    onSubmit,
     placeholder,
   }: {
     value: string;
     onChange: (s: string) => void;
+    onSubmit: () => void;
     placeholder: string;
   }) => (
-    <textarea aria-label={placeholder} value={value} onChange={(e) => onChange(e.target.value)} />
+    <textarea
+      aria-label={placeholder}
+      value={value}
+      onChange={(e) => onChange(e.target.value)}
+      onKeyDown={(e) => {
+        if (e.key !== "Enter") return;
+        e.preventDefault();
+        onSubmit();
+      }}
+    />
   ),
   ModelMenu: ({
     value,
@@ -199,7 +210,7 @@ describe("WorkspaceHome", () => {
     await user.type(screen.getByLabelText("How can I help you today?"), "fix the login");
     await user.click(screen.getByTestId("workspace-home-start"));
     expect(api.lanes.action).toHaveBeenCalledWith(1, "start", {
-      slot: 1,
+      slot: "auto",
       prompt: "fix the login",
       permissionMode: "auto",
     });
@@ -248,7 +259,7 @@ describe("WorkspaceHome", () => {
     await user.type(screen.getByLabelText("How can I help you today?"), "port it");
     await user.click(screen.getByTestId("workspace-home-start"));
     expect(api.lanes.action).toHaveBeenCalledWith(1, "start", {
-      slot: 1,
+      slot: "auto",
       prompt: "port it",
       tool: "codex",
       sandbox: "workspace-write",
@@ -300,23 +311,40 @@ describe("WorkspaceHome", () => {
     await user.type(screen.getByLabelText("How can I help you today?"), "try it");
     await user.click(screen.getByTestId("workspace-home-start"));
     expect(api.lanes.action).toHaveBeenCalledWith(1, "start", {
-      slot: 1,
+      slot: "auto",
       prompt: "try it",
       permissionMode: "auto",
       worktree: true,
     });
   });
 
-  it("refuses to start when all four panes are busy", async () => {
-    runs = [0, 1, 2, 3].map(
-      (slot) => ({ id: `r${slot}`, laneId: 1, 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();
     renderHome();
     await screen.findByTestId("workspace-recent-s-old");
+    await user.type(screen.getByLabelText("How can I help you today?"), "go");
     await user.click(screen.getByTestId("workspace-home-start"));
-    expect(screen.getByRole("alert")).toHaveTextContent("All four panes");
+    expect(await screen.findByRole("alert")).toHaveTextContent("All four panes");
+  });
+
+  it("starts nothing on an empty prompt, and only once on a double Enter", async () => {
+    let release: (v: { run_id: string }) => void = () => undefined;
+    vi.mocked(api.lanes.action).mockImplementationOnce(
+      () => new Promise((r) => (release = r)) as never
+    );
+    const user = userEvent.setup();
+    renderHome();
+    await screen.findByTestId("workspace-recent-s-old");
+    const box = screen.getByLabelText("How can I help you today?");
+    expect(screen.getByTestId("workspace-home-start")).toBeDisabled();
+    await user.type(box, "{Enter}");
     expect(api.lanes.action).not.toHaveBeenCalled();
+    await user.type(box, "go{Enter}{Enter}");
+    release({ run_id: "agenthub-lane-1-1" });
+    await waitFor(() => expect(api.lanes.action).toHaveBeenCalledTimes(1));
   });
 
   it("folds the archived sessions with Unarchive", async () => {