#6630: feat(hooks): add agent:turn_start and agent:turn_end lifecycle events

Add two new hook events that fire when the agent begins and finishes processing an inbound message: - agent:turn_start: Fires before the agent processes a message - agent:turn_end: Fires after the agent sends a response (in finally block) Both events include context about the message source, sender, and a preview of the message content. This enables integrations like status dashboards to show real-time agent activity. Example use case: A Mission Control dashboard that shows when the agent is actively working vs idle. Implements planned events from docs/hooks.md (replaces message:sent and message:received with more precise turn-based events). <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR adds two new internal hook events (`agent:turn_start` and `agent:turn_end`) emitted from the inbound message dispatch path, and documents their context payload fields in `docs/hooks.md`. The intent is to provide a turn-based lifecycle signal (start/end of processing) for integrations like dashboards. Main concern: the `turn_end` event is emitted from a `finally` block but currently hardcodes `success: true`, so failures will be reported as successes. Additionally, `turn_start` is awaited before the main dispatch, so any unexpected failure in the hook trigger path could block message processing. The docs update is helpful, but it still lists `message:sent`/`message:received` under “Future Events” despite the PR describing them as replaced by the new turn events. <h3>Confidence Score: 3/5</h3> - Reasonably safe to merge, but the new hook telemetry semantics are currently incorrect for error cases. - Changes are small and localized, but `agent:turn_end` always sets `success: true`, which can mislead downstream consumers and is likely a bug for the intended API. The hook triggers are also on the critical dispatch path, so any unexpected failure there could impact message processing. - src/auto-reply/dispatch.ts <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->