Updates the kwarg passed to mention handlers from "mention" to "context"
to better reflect that it contains a MentionContext object with comment,
triggered_by, and scope fields. Also removes unused _mention_permission
attribute from MentionHandlerBase protocol.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Moves the get_event_scope function to be a classmethod on MentionScope
called from_event, following the same pattern as Comment.from_event.
This provides a more consistent API and better encapsulation.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

- Add explicit kwargs to mention() decorator (pattern, username, scope)
- Pass context as explicit parameter instead of mutating kwargs
- Create MentionEvent.from_event() generator to encapsulate all mention
  processing logic (parsing, filtering, context creation)
- Rename MentionContext to MentionEvent for clarity
- Simplify router code from ~30 lines to 4 lines per wrapper

This creates a much cleaner API where the mention decorator's wrappers
simply iterate over MentionEvent instances yielded by the generator.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

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…ngo-github-app into gh-command

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