Add option to clean up resolved comments

## Summary

There's currently no easy way to clean up resolved comments/annotations in the Changes/Diff view. Once a comment thread is resolved, it remains visible and clutters the review surface, especially on larger PRs or after multiple review passes.

## Proposed

Add an action to bulk-clean up (hide or remove) resolved comments. Some options worth considering:

- A "Clear resolved comments" button in the Changes/Diff view toolbar
- A filter toggle to hide resolved threads (with the option to bring them back)
- A per-file or whole-PR scope

## Why

- Keeps the review surface focused on open/unresolved feedback
- Reduces visual noise after iterating on a PR
- Matches behavior users expect from other code-review tools

Happy to discuss the exact UX (hide vs. delete, persistence across sessions, etc.).