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/store/chat/streamStore.ts

@@ -382,6 +382,35 @@ export const useStreamStore = create<StreamState>()(
               }
             }
           },
+          onSubtaskStatus: (description: string, status: string, detail?: string) => {
+            // Map backend status to frontend Subtask status
+            const statusMap: Record<string, Subtask['status']> = {
+              started: 'pending',
+              running: 'in_progress',
+              completed: 'completed',
+              failed: 'failed',
+            };
+            const mappedStatus = statusMap[status] || 'in_progress';
+
+            _chat?.updateMessages(msgs =>
+              msgs.map(m => {
+                if (m.id !== assistantId) return m;
+                const subtasks = [...(m.subtasks || [])];
+                const existingIdx = subtasks.findIndex(st => st.description === description);
+                if (existingIdx >= 0) {
+                  subtasks[existingIdx] = { ...subtasks[existingIdx], status: mappedStatus, result: detail };
+                } else {
+                  subtasks.push({
+                    id: `subtask_${Date.now()}_${generateRandomString(4)}`,
+                    description,
+                    status: mappedStatus,
+                    result: detail,
+                  });
+                }
+                return { ...m, subtasks };
+              })
+            );
+          },
           onComplete: (inputTokens?: number, outputTokens?: number) => {
             const currentMsgs = _chat?.getMessages();