Fix: no emitir deltas del planner/reviewer al frontend

El planner genera JSON interno que no debe mostrarse al usuario.
Solo coder y collector emiten AGENT_DELTA al stream.

Para direct_response, el engine emite como agent=coder para que
el ClaudeFormatEmitter lo procese correctamente.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/orchestrator/engine.py

@@ -129,7 +129,7 @@ class OrchestratorEngine:
                 # Emit as text streaming for the frontend
                 await self.sse.emit(
                     EventType.AGENT_DELTA,
-                    {"agent": "planner", "delta": plan_result, "step": 0},
+                    {"agent": "coder", "delta": plan_result, "step": 0},
                     session_id=session.session_id,
                 )