Split ParsedGitHubContext into entity-specific and automation contexts:
- ParsedGitHubContext: For entity events (issues/PRs) with required entityNumber and isPR
- AutomationContext: For workflow_dispatch/schedule events without entity fields
- GitHubContext: Union type for all contexts
This eliminates ~20 null checks throughout the codebase and provides better type safety.
Entity-specific code paths are now guaranteed to have the required fields.
Co-Authored-By: Claude <noreply@anthropic.com>