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__/LaneChangesPanel.test.tsx

@@ -15,9 +15,15 @@ import { render, screen, waitFor, act, within } from "@testing-library/react";
 import userEvent from "@testing-library/user-event";
 import { useState } from "react";
 import { LaneChangesPanel } from "../LaneChangesPanel";
+import { confirmDiscard } from "../../../lib/confirm";
 import { api } from "../../../lib/api";
 import type { LaneChanges } from "../../../lib/types";
 
+// The themed confirm needs its host (App); answer it here.
+vi.mock("../../../lib/confirm", () => ({
+  confirmDiscard: vi.fn(async () => true),
+  confirmDialog: vi.fn(async () => true),
+}));
 vi.mock("../../../lib/api", () => ({
   gitTarget: (lane: number, session?: string | null) => (session ? { lane, session } : lane),
   api: {
@@ -134,7 +140,13 @@ beforeEach(() => {
 
 /** The Workspace right panel's Files/Changes activity-bar buttons, around the
  *  panel — the tab is the Workspace's state, passed in as `tab`. */
-function Tabbed({ laneId }: { laneId: number }) {
+function Tabbed({
+  laneId,
+  onDirtyChange,
+}: {
+  laneId: number;
+  onDirtyChange?: (dirty: boolean) => void;
+}) {
   const [tab, setTab] = useState<"files" | "changes">("changes");
   return (
     <>
@@ -144,7 +156,7 @@ function Tabbed({ laneId }: { laneId: number }) {
       <button type="button" onClick={() => setTab("changes")}>
         Changes
       </button>
-      <LaneChangesPanel laneId={laneId} tab={tab} />
+      <LaneChangesPanel laneId={laneId} tab={tab} onDirtyChange={onDirtyChange} />
     </>
   );
 }
@@ -404,7 +416,12 @@ describe("LaneChangesPanel", () => {
     expect(screen.getByTestId("lane-file-save")).toBeDisabled();
     await user.type(editor, "x");
     expect(screen.getByTestId("lane-file-save")).toBeEnabled();
-    // Cancel drops the edit and goes back to reading.
+    // Cancel asks before dropping an edit: "keep" keeps it…
+    vi.mocked(confirmDiscard).mockClear().mockResolvedValueOnce(false);
+    await user.click(screen.getByRole("button", { name: "Cancel" }));
+    expect(confirmDiscard).toHaveBeenCalledTimes(1);
+    expect(screen.getByTestId("lane-file-editor")).toHaveValue("const whole = 1;x");
+    // …and "discard" drops it and goes back to reading.
     await user.click(screen.getByRole("button", { name: "Cancel" }));
     expect(await screen.findByTestId("lane-file-view")).toHaveTextContent("const whole = 1;");
     expect(screen.queryByTestId("lane-file-editor")).toBeNull();
@@ -739,6 +756,33 @@ describe("LaneChangesPanel", () => {
     expect(screen.queryByTestId("lane-file-editor")).toBeNull();
   });
 
+  // The Workspace asked before switching tab; the edit must then be GONE, or
+  // `dirty` stays true and keeps the panel pinned and the unload prompt on.
+  it("drops an edited file when the tab switches away from it", async () => {
+    vi.mocked(api.lanes.gitFile).mockResolvedValue({
+      available: true,
+      path: "src/main.ts",
+      rev: "work",
+      binary: false,
+      truncated: false,
+      lines: ["const whole = 1;"],
+      sha: "b".repeat(40),
+    });
+    const dirty = vi.fn();
+    const user = userEvent.setup();
+    render(<Tabbed laneId={7} onDirtyChange={dirty} />);
+    await screen.findByTestId("lane-changes-overview");
+    await user.click(screen.getByRole("button", { name: "Files" }));
+    await user.type(await screen.findByLabelText("Find a file by path…"), "main");
+    await user.click(await screen.findByText("src/main.ts"));
+    await user.click(await screen.findByTestId("lane-file-view"));
+    await user.type(await screen.findByTestId("lane-file-editor"), "x");
+    expect(dirty).toHaveBeenLastCalledWith(true);
+    await user.click(screen.getByRole("button", { name: "Changes" }));
+    await waitFor(() => expect(dirty).toHaveBeenLastCalledWith(false));
+    expect(screen.queryByTestId("lane-file-editor")).toBeNull();
+  });
+
   it("says why a file cannot be edited instead of offering a dead Edit button", async () => {
     vi.mocked(api.lanes.gitFile).mockResolvedValue({
       available: true,