feat(enrichment): revert-recurrence detector for review brief

[kiannidev]

Summary

Add a REES analyzer that detects when a PR is reverting or re-introducing previously reverted work — a common source of regressions and review churn.

Behavior

The analyzer inspects PR title/body and the diff for revert signals:

• Explicit revert language (revert, rollback, undo, reverts commit)
• GitHub-style revert titles (Revert "…")
• High deletion-to-addition ratio on files that were recently changed in the same direction

Findings are rendered in the external review brief so the AI reviewer can flag regression risk.

Scope

• New review-enrichment/src/analyzers/revert-recurrence.ts
• Wire into brief.ts, types.ts, render.ts
• Unit tests under review-enrichment/test/

Acceptance criteria

• Detects explicit revert titles/bodies
• Flags symmetric churn patterns in diff hunks
• Fail-safe: returns [] on empty input
• Brief renders a dedicated revert-recurrence section when findings exist