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/core/hooks/index.ts

@@ -12,6 +12,8 @@ export { wrapToolsWithHooks, wrapToolWithHooks } from "./tool-wrapper.js";
 export type {
 	AgentEndEvent,
 	AgentStartEvent,
+	BeforeAgentStartEvent,
+	BeforeAgentStartEventResult,
 	BashToolResultEvent,
 	ContextEvent,
 	ContextEventResult,