feat: sub-agent streaming progress — TaskTool emits real-time status events

- Rust: LoopEvent::SubtaskStatus variant added to loop_runner.rs
- Rust: ToolContext.event_sender field for streaming tool progress
- Rust: TaskTool emits started/running/completed/failed via event_sender
- Rust: StreamChatEvent::SubtaskStatus mapped in Tauri chat command
- TS: StreamEventSubtaskStatus type + onSubtaskStatus callback added
- TS: kernel-chat.ts handles subtaskStatus event from Tauri
- TS: streamStore.ts wires callback, maps backend→frontend status,
  updates assistant message subtasks array in real-time

desktop/src/lib/kernel-chat.ts

@@ -146,6 +146,17 @@ export function installChatMethods(ClientClass: { prototype: KernelClient }): vo
             }
             break;
 
+          case 'subtaskStatus':
+            log.debug('Subtask status:', streamEvent.description, streamEvent.status, streamEvent.detail);
+            if (callbacks.onSubtaskStatus) {
+              callbacks.onSubtaskStatus(
+                streamEvent.description,
+                streamEvent.status,
+                streamEvent.detail ?? undefined
+              );
+            }
+            break;
+
           case 'iterationStart':
             log.debug('Iteration started:', streamEvent.iteration, '/', streamEvent.maxIterations);
             // Don't need to notify user about iterations

desktop/src/lib/kernel-types.ts

@@ -69,6 +69,7 @@ export interface StreamCallbacks {
   onThinkingDelta?: (delta: string) => void;
   onTool?: (tool: string, input: string, output: string) => void;
   onHand?: (name: string, status: string, result?: unknown) => void;
+  onSubtaskStatus?: (description: string, status: string, detail?: string) => void;
   onComplete: (inputTokens?: number, outputTokens?: number) => void;
   onError: (error: string) => void;
 }
@@ -126,6 +127,13 @@ export interface StreamEventHandEnd {
   result: unknown;
 }
 
+export interface StreamEventSubtaskStatus {
+  type: 'subtaskStatus';
+  description: string;
+  status: string;
+  detail?: string;
+}
+
 export type StreamChatEvent =
   | StreamEventDelta
   | StreamEventThinkingDelta
@@ -134,6 +142,7 @@ export type StreamChatEvent =
   | StreamEventIterationStart
   | StreamEventHandStart
   | StreamEventHandEnd
+  | StreamEventSubtaskStatus
   | StreamEventComplete
   | StreamEventError;