feat(coding-agent): add event bus for tool/hook communication (#431)

* feat(coding-agent): add event bus for tool/hook communication

Adds pi.events API enabling custom tools and hooks to communicate via
pub/sub. Tools can emit events, hooks can listen. Shared EventBus instance
created per session in createAgentSession().

- EventBus interface with emit() and on() methods
- on() returns unsubscribe function
- Threaded through hook and tool loaders
- Documented in hooks.md and custom-tools.md

* fix(coding-agent): wrap event handlers to catch errors

* docs: note async handler error handling for event bus

* feat(coding-agent): add sendMessage to tools, nextTurn delivery mode

- Custom tools now have pi.sendMessage() for direct agent notifications
- New deliverAs: 'nextTurn' queues messages for next user prompt
- Fix: hooks and tools now share the same eventBus (was isolated before)

* fix(coding-agent): nextTurn delivery should always queue, even when streaming

packages/coding-agent/CHANGELOG.md

@@ -43,6 +43,9 @@
 - Tool registry now contains all built-in tools (read, bash, edit, write, grep, find, ls) even when `--tools` limits the initially active set. Hooks can enable any tool from the registry via `pi.setActiveTools()`.
 - System prompt now automatically rebuilds when tools change via `setActiveTools()`, updating tool descriptions and guidelines to match the new tool set
 - Hook errors now display full stack traces for easier debugging
+- Event bus (`pi.events`) for tool/hook communication: shared pub/sub between custom tools and hooks
+- Custom tools now have `pi.sendMessage()` to send messages directly to the agent session without needing the event bus
+- `sendMessage()` supports `deliverAs: "nextTurn"` to queue messages for the next user prompt
 
 ### Changed