AI Agent: Implement code review loop with GitHub PR integration #15

Problem

Our AI agent can search issues and create issues, but it has no awareness of code changes. When a PR is opened, the agent should be able to:

1. Read the diff
2. Understand which issues the PR relates to
3. Post a structured review summary
4. Flag potential issues (missing tests, breaking changes, schema conflicts)

Why This Matters

This is the 'code-aware orchestration' loop that differentiates us from pure issue trackers (Linear) and pure code reviewers (CodeRabbit). We connect both worlds.

Acceptance Criteria

• Agent tool: fetchPRDiff(repoName, prNumber) -> returns file changes
• Agent tool: analyzePRImpact() -> maps changed files to related issues
• Agent can post a review summary as a comment on the issue
• Works within the existing rate limiter (counts as 1 agent interaction)
• Gate behind ai_agent feature flag