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/LaneGitHeader.tsx

@@ -267,7 +267,13 @@ export function LaneGitHeader({
           // keyboard or a touchscreen has.
           onFocus={openFiles}
           onBlur={closeFiles}
-          onClick={() => setFilesOpen((v) => !v)}
+          // Click only OPENS: hover (or a tap's focus) has already opened it,
+          // and a toggle here shut it again on the same gesture. Leaving,
+          // blurring or Escape closes it.
+          onClick={openFiles}
+          onKeyDown={(e) => {
+            if (e.key === "Escape") setFilesOpen(false);
+          }}
           className="shrink-0 cursor-help rounded-full border border-status-warning/40 bg-status-warning/10 px-2 py-px text-[10.5px] text-status-warning"
         >
           {t("git.uncommitted", { dirty: git.dirty, untracked: git.untracked })}