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/lib/__tests__/panePaste.test.ts

@@ -2,7 +2,8 @@
  * @file panePaste.test.ts
  * @description Pins where a pasted diff comment lands: the workspace's pane
  * focused last, else any of its panes, never another workspace's (lane 1 is
- * not lane 12), and nowhere once the pane unmounted.
+ * not lane 12), nowhere once the pane unmounted, and never into a pane that
+ * is not on screen (a hidden console behind a full-width Changes panel).
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
@@ -16,7 +17,7 @@ describe("panePaste", () => {
     const off12 = reg("agenthub-lane-12");
     const off0 = reg("agenthub-lane-1");
     const off2 = reg("agenthub-lane-1-2");
-    expect(pasteToLane(1, "a")).toBe(true);
+    expect(pasteToLane(1, "a")).toBe("agenthub-lane-1");
     expect(got["agenthub-lane-1"]).toEqual(["a"]);
     notePaneFocus("agenthub-lane-1-2");
     pasteToLane(1, "b");
@@ -27,7 +28,24 @@ describe("panePaste", () => {
     expect(got["agenthub-lane-12"]).toBeUndefined();
     off0();
     off2();
-    expect(pasteToLane(1, "d")).toBe(false);
+    expect(pasteToLane(1, "d")).toBeNull();
     off12();
   });
+
+  it("never pastes into a pane that is not visible", () => {
+    const got: string[] = [];
+    let shown = false;
+    const off = registerPane(
+      "agenthub-lane-5",
+      (t) => got.push(t),
+      () => shown
+    );
+    notePaneFocus("agenthub-lane-5");
+    expect(pasteToLane(5, "x")).toBeNull();
+    expect(got).toEqual([]);
+    shown = true;
+    expect(pasteToLane(5, "y")).toBe("agenthub-lane-5");
+    expect(got).toEqual(["y"]);
+    off();
+  });
 });