feat(sagi): spawn a real claude session for Sagi's Ask handoff

Unmatched Ask questions now adopt a dedicated "Sagi" lane (POST
/api/lanes/ensure, cwd defaults to $HOME) and spawn a tmux-backed claude
run there instead of just prefilling the Run page composer.

client/src/components/Sagi/useSagiBrain.ts

@@ -87,6 +87,10 @@ export interface SagiBrain {
   status: SagiStatus;
   bubble: string | null;
   dismissBubble: () => void;
+  /** Imperative bubble setter for one-off messages from the shell (e.g. spawn
+   *  errors). Bypasses the throttle because the caller decided this message
+   *  must be seen right now. Auto-clears after the standard bubble TTL. */
+  setBubble: (text: string) => void;
   muted: boolean;
   toggleMute: () => void;
   clearAlerts: () => void;
@@ -103,7 +107,7 @@ export function useSagiBrain(): SagiBrain {
     connected: true,
   }));
   const [tick, setTick] = useState(now0);
-  const [bubble, setBubble] = useState<string | null>(null);
+  const [bubble, setBubbleState] = useState<string | null>(null);
   const [muted, setMuted] = useState<boolean>(() => sagiPrefs.getMuted());
 
   const bubbleTimer = useRef<ReturnType<typeof setTimeout>>();
@@ -121,8 +125,8 @@ export function useSagiBrain(): SagiBrain {
     if (!force && t - lastBubbleAt.current < BUBBLE_THROTTLE_MS) return;
     lastBubbleAt.current = t;
     clearTimeout(bubbleTimer.current);
-    setBubble(text);
-    bubbleTimer.current = setTimeout(() => setBubble(null), BUBBLE_MS);
+    setBubbleState(text);
+    bubbleTimer.current = setTimeout(() => setBubbleState(null), BUBBLE_MS);
   }, []);
 
   // Seed from the REST snapshot on mount so counts are accurate immediately -
@@ -176,7 +180,7 @@ export function useSagiBrain(): SagiBrain {
 
   const dismissBubble = useCallback(() => {
     clearTimeout(bubbleTimer.current);
-    setBubble(null);
+    setBubbleState(null);
   }, []);
 
   const toggleMute = useCallback(() => {
@@ -193,5 +197,25 @@ export function useSagiBrain(): SagiBrain {
     []
   );
 
-  return { mood, status, bubble, dismissBubble, muted, toggleMute, clearAlerts, setThinking };
+  // One-shot bubble: clears itself after BUBBLE_MS. Used by the shell for
+  // imperative messages (e.g. spawn errors) that must appear now and fade,
+  // bypassing the throttled mood-driven showBubble path.
+  const setBubble = useCallback((text: string) => {
+    if (!text) return;
+    clearTimeout(bubbleTimer.current);
+    setBubbleState(text);
+    bubbleTimer.current = setTimeout(() => setBubbleState(null), BUBBLE_MS);
+  }, []);
+
+  return {
+    mood,
+    status,
+    bubble,
+    dismissBubble,
+    setBubble,
+    muted,
+    toggleMute,
+    clearAlerts,
+    setThinking,
+  };
 }