fix(reviewer-bot): prefer current-head reviews and track thread replies

[PLeVasseur]

Summary

• prefer current-head reviewer reviews over newer stale-head reviews when projecting reviewer-response state, and refresh stored reviewer-review state during reconcile and sweeper repair
• add deferred observer, reconcile, and sweeper coverage for pull_request_review_comment so review-thread replies update freshness state and diagnostics
• add regression coverage for the #368 stale-review case, the #256 preservation case, and review-comment workflow routing

Testing

• uv run ruff check --fix
• uv run pytest .github/reviewer-bot-tests

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