Add PR review comments to pull request dialog

[blindndangerous]

Summary

• Include PR review comments in the pull request dialog, not only issue-style PR comments.
• Merge both comment streams in chronological order so PR comment history reads naturally.
• Label review-thread entries in the list as [Review] to make comment source clear.

What Changed

• Added get_pr_review_comments(...) in github_api.py using GET /repos/{owner}/{repo}/pulls/{number}/comments.
• Updated get_pr_comments(...) to combine issue comments and review comments, then sort by created_at.
• Extended models.issue.Comment with a kind field (issue or
  eview).
• Updated PR dialog list rendering to prefix review comments with [Review].

Testing Status

• Draft PR: functional user testing is still pending because there is not currently a reliable test case in hand for incoming PR review-comment activity.
• Build was produced successfully from this branch for manual testing.

Stacked PR Notes

• Depends on Backfill missing PR and issue titles in activity feed #3 (which depends on Add native discussion dialog support for feed and notifications #2, which depends on Improve activity feed text for issue/PR events #1).
• After lower stacked PRs merge, this PR will be the PR-review-comment delta.

Attribution

• Implemented by an AI agent: OpenAI Codex (GPT-5).
• Pending additional human validation via app build.