fix: resolved review threads don't trigger can-be-merged re-evaluation

[myakove]

Problem

When a review thread is resolved on a PR, can-be-merged is not re-evaluated. The PR stays in a stale "cannot be merged: unresolved conversations" state until another event (like a check run or status update) triggers re-evaluation.

Root Cause

GitHub sends a pull_request_review_thread webhook event with action resolved/unresolved, but the webhook server does not handle this event type. The process() method in github_api.py only handles: ping, push, issue_comment, pull_request, pull_request_review, check_run, status.

Expected Behavior

When a review thread is resolved or unresolved, can-be-merged should be re-evaluated to reflect the current state of conversations.

Deliverables

• Add pull_request_review_thread event handler in github_api.py process()
• Re-evaluate can-be-merged on resolved and unresolved actions
• Lightweight handler (no clone needed — conversation check uses GraphQL API)
• Add tests for resolved/unresolved thread handling