Implement before_agent_start hook event

- Add BeforeAgentStartEvent and BeforeAgentStartEventResult types
- Add emitBeforeAgentStart to HookRunner
- Call in AgentSession.prompt() before agent.prompt()
- Hook can return a message to inject into context (persisted + visible)
- Add test hook demonstrating custom message rendering and before_agent_start

packages/coding-agent/src/index.ts

@@ -42,6 +42,8 @@ export type {
 	AgentEndEvent,
 	AgentStartEvent,
 	BashToolResultEvent,
+	BeforeAgentStartEvent,
+	BeforeAgentStartEventResult,
 	CustomToolResultEvent,
 	EditToolResultEvent,
 	FindToolResultEvent,