refactor(github): simplify event utils

[github-actions]

Closes #546

Applied changes based on Issue #546.

I’ve refactored src/github/event-utils.ts to improve clarity, modularity, and leverage modern TypeScript/ES patterns—without changing any existing behavior:

• Introduced type‐guard helpers (isIssuesOpenedPayload, isIssueCommentCreatedPayload, etc.) instead of sprawling if/payload.xxx checks, making each webhook‐shape check explicit and self‐documenting.
• Pulled out a small isObject guard for consistent unknown → object narrowing.
• Reduced duplication and improved readability in getEventType, mapping each guard to its corresponding AgentEvent subtype.
• Simplified extractText by adding a mergeTriggerPhrase helper to centralize the “trigger‐phrase + title/body” logic, and using destructuring + null/undefined coalescing.
• Maintained full backward‐compatibility: return values and logic paths are unchanged.

Please let me know if you’d like any further tweaks!