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/run/LaneConsolePane.tsx

@@ -102,7 +102,12 @@ export function LaneConsolePane({
     autoAttachedForLane.current = laneId;
     api.run
       .get(attachRequest.runId)
-      .then((h) => live && setHandle(h))
+      .then((h) => {
+        if (!live) return;
+        setHandle(h);
+        // A later attach that worked clears an earlier one's failure.
+        setError(null);
+      })
       .catch(
         (err: unknown) =>
           live &&
@@ -136,7 +141,7 @@ export function LaneConsolePane({
     <div data-testid="console-body" className="flex min-h-0 flex-1 flex-col gap-3">
       {/* Not probed yet (a tool the page has not probed, or a shell) is not "missing". */}
       {binaryStatus?.[paneTool] && !binaryStatus[paneTool].found && (
-        <div className="rounded-lg border border-status-danger/40 bg-status-danger/10 px-4 py-3 text-sm text-status-danger flex items-center gap-2">
+        <div className="mx-3 mt-2 rounded-lg border border-status-danger/40 bg-status-danger/10 px-4 py-3 text-sm text-status-danger flex items-center gap-2">
           <AlertCircle className="w-4 h-4 flex-shrink-0" />
           <span>
             {t("binary.missing", {
@@ -148,7 +153,7 @@ export function LaneConsolePane({
       )}
 
       {error && (
-        <div className="rounded-lg border border-status-danger/40 bg-status-danger/10 px-4 py-3 text-sm text-status-danger flex items-center gap-2">
+        <div className="mx-3 mt-2 rounded-lg border border-status-danger/40 bg-status-danger/10 px-4 py-3 text-sm text-status-danger flex items-center gap-2">
           <AlertCircle className="w-4 h-4 flex-shrink-0" />
           <span className="flex-1 break-all">{error}</span>
         </div>
@@ -157,7 +162,7 @@ export function LaneConsolePane({
       {!handle ? (
         <div
           data-testid="pane-no-run"
-          className="flex flex-1 flex-col items-center justify-center gap-3 rounded-xl border border-dashed border-border p-6 text-center text-sm text-fg-muted"
+          className="m-3 flex flex-1 flex-col items-center justify-center gap-3 rounded-xl border border-dashed border-border p-6 text-center text-sm text-fg-muted"
         >
           <p>{t("emptyPane")}</p>
           {onNewSession && (