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-tauri/src/kernel_commands/chat.rs

@@ -37,6 +37,7 @@ pub enum StreamChatEvent {
     ThinkingDelta { delta: String },
     ToolStart { name: String, input: serde_json::Value },
     ToolEnd { name: String, output: serde_json::Value },
+    SubtaskStatus { description: String, status: String, detail: Option<String> },
     IterationStart { iteration: usize, max_iterations: usize },
     HandStart { name: String, params: serde_json::Value },
     HandEnd { name: String, result: serde_json::Value },
@@ -294,6 +295,14 @@ pub async fn agent_chat_stream(
                                 StreamChatEvent::ToolEnd { name: name.clone(), output: output.clone() }
                             }
                         }
+                        LoopEvent::SubtaskStatus { description, status, detail } => {
+                            tracing::debug!("[agent_chat_stream] SubtaskStatus: {} - {}", description, status);
+                            StreamChatEvent::SubtaskStatus {
+                                description: description.clone(),
+                                status: status.clone(),
+                                detail: detail.clone(),
+                            }
+                        }
                         LoopEvent::IterationStart { iteration, max_iterations } => {
                             tracing::debug!("[agent_chat_stream] IterationStart: {}/{}", iteration, max_iterations);
                             StreamChatEvent::IterationStart { iteration: *iteration, max_iterations: *max_iterations }