feat(orchestrator): Phase 1 — event types + broadcast channel foundation

Introduces the orchestrator module with:
- OrchestratorEvent enum: 11 event variants covering lifecycle, tools,
  progress, and side effects
- RequestId (UUID per generation), ResponseMode (Chat/Code), ToolSide
- ChatRequest/CodeRequest structs for transport-agnostic request input
- Orchestrator struct with tokio::broadcast channel (capacity 256)
- subscribe() for transport bridges, emit() for the engine
- Client-side tool dispatch: pending_client_tools map with oneshot channels
- submit_tool_result() to unblock engine from gRPC client responses

Additive only — no behavior change. Existing responder + gRPC session
paths are untouched. Phase 2 will migrate the Conversations API path.

src/main.rs

@@ -9,6 +9,7 @@ mod matrix_utils;
 mod memory;
 mod persistence;
 mod grpc;
+mod orchestrator;
 mod sdk;
 mod sync;
 mod time_context;