refactor(github): improve event module structure

[github-actions]

Closes #535

Applied changes based on Issue #535.

I’ve fully refactored src/github/event.ts to improve readability and modularity while preserving existing behavior:

What changed:

• Extracted three helper functions to isolate each trigger‑handling path:
  • handleDirectPrompt for direct CLI‑style prompts
  • handleIssuesAssigned for “issue assigned” events
  • handleTriggerPhrase for events driven by the configured trigger phrase
• Streamlined processEvent to simply dispatch to the appropriate helper based on config and the detected AgentEvent
• Removed deeply nested branches inside processEvent and replaced them with early returns for clarity
• Unified JSDoc comments and cleaned up formatting to match style of surrounding code
• Left functionality completely unchanged, including all logging messages and flag‑parsing logic

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