feat(hooks): add text_delta event for streaming text monitoring

- New text_delta hook event fires for each chunk of streaming text
- Enables real-time monitoring of agent output
- Plan-mode hook now updates todo progress as [DONE:id] tags stream in
- Each todo item has unique ID for reliable tracking

packages/coding-agent/src/core/agent-session.ts

@@ -224,6 +224,15 @@ export class AgentSession {
 
 	/** Internal handler for agent events - shared by subscribe and reconnect */
 	private _handleAgentEvent = async (event: AgentEvent): Promise<void> => {
+		// Emit text_delta events to hooks for streaming text monitoring
+		if (
+			event.type === "message_update" &&
+			event.assistantMessageEvent.type === "text_delta" &&
+			this._hookRunner
+		) {
+			await this._hookRunner.emit({ type: "text_delta", text: event.assistantMessageEvent.delta });
+		}
+
 		// When a user message starts, check if it's from either queue and remove it BEFORE emitting
 		// This ensures the UI sees the updated queue state
 		if (event.type === "message_start" && event.message.role === "user") {

packages/coding-agent/src/core/hooks/types.ts

@@ -392,6 +392,16 @@ export interface AgentEndEvent {
 	messages: AgentMessage[];
 }
 
+/**
+ * Event data for text_delta event.
+ * Fired when new text is streamed from the assistant.
+ */
+export interface TextDeltaEvent {
+	type: "text_delta";
+	/** The new text chunk */
+	text: string;
+}
+
 /**
  * Event data for turn_start event.
  */
@@ -535,6 +545,7 @@ export type HookEvent =
 	| BeforeAgentStartEvent
 	| AgentStartEvent
 	| AgentEndEvent
+	| TextDeltaEvent
 	| TurnStartEvent
 	| TurnEndEvent
 	| ToolCallEvent
@@ -701,6 +712,7 @@ export interface HookAPI {
 	on(event: "turn_end", handler: HookHandler<TurnEndEvent>): void;
 	on(event: "tool_call", handler: HookHandler<ToolCallEvent, ToolCallEventResult>): void;
 	on(event: "tool_result", handler: HookHandler<ToolResultEvent, ToolResultEventResult>): void;
+	on(event: "text_delta", handler: HookHandler<TextDeltaEvent>): void;
 
 	/**
 	 * Send a custom message to the session. Creates a CustomMessageEntry that