feat(cli): add A2A HTTP listener for external message injection in Forever Mode

Embed a JSON-RPC 2.0 HTTP server that bridges A2A protocol messages into
the interactive session. Starts automatically in Forever Mode, binds to
127.0.0.1 on a configurable port (sisyphusMode.a2aPort), and writes a
port discovery file to ~/.gemini/sessions/.

Supported methods: message/send (blocking), tasks/get, responses/poll,
and GET /.well-known/agent-card.json.

- Add ExternalMessage and A2AListenerStarted app events
- Track streaming state transitions to capture agent responses
- Display A2A port in StatusDisplay when active

packages/cli/src/ui/contexts/UIStateContext.tsx

@@ -223,6 +223,7 @@ export interface UIState {
   showIsExpandableHint: boolean;
   hintMode: boolean;
   hintBuffer: string;
+  a2aListenerPort: number | null;
   transientMessage: {
     text: string;
     type: TransientMessageType;