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/i18n/locales/en/run.json

@@ -148,5 +148,17 @@
     "imagesOnly": "Only images can be dropped here",
     "remove": "Remove image",
     "view": "View image"
+  },
+  "terminal": {
+    "ended": "[session ended, exit code {{code}}]",
+    "reconnecting": "Reconnecting… keystrokes are not reaching the session until the terminal is back.",
+    "keyboardHint": "Terminal. Press Ctrl+M to toggle whether Tab moves focus out of the terminal; Ctrl+Shift+F (Cmd+F on macOS) finds text.",
+    "tabMoves": "Tab moves focus — Ctrl+M to give Tab back to the terminal",
+    "selectHint": "Shift+drag to select text in the browser; a plain drag selects in tmux and copies on release.",
+    "dismissHint": "Dismiss",
+    "find": "Find in terminal",
+    "findPrev": "Previous match",
+    "findNext": "Next match",
+    "findClose": "Close find"
   }
 }