feat(engine): live progress status updates via event broadcast

Engine v2 now shows live progress in the CLI (and any channel):
- "Thinking..." when a step starts
- Tool name + success/error when actions execute
- "Processing results..." when a step completes

Implementation:
- ThreadManager holds a broadcast::Sender<ThreadEvent> (capacity 256)
- ExecutionLoop.emit_event() writes to thread.events AND broadcasts
- ThreadManager.subscribe_events() returns a receiver
- Router uses tokio::select! to listen for events while waiting for
  thread completion, forwarding them as StatusUpdate to the channel

This replaces the polling approach with zero-latency event streaming.
Agent.channels visibility widened to pub(crate) for bridge access.

102 tests passing, zero clippy warnings.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

src/agent/agent_loop.rs

@@ -175,7 +175,7 @@ pub struct AgentDeps {
 pub struct Agent {
     pub(super) config: AgentConfig,
     pub(super) deps: AgentDeps,
-    pub(super) channels: Arc<ChannelManager>,
+    pub(crate) channels: Arc<ChannelManager>,
     pub(super) context_manager: Arc<ContextManager>,
     pub(super) scheduler: Arc<Scheduler>,
     pub(super) router: Router,