fix(review): restore issue comment classification, add PR deep review as separate mode

Revises implementation based on PR feedback:
- Keep original issue comment classification (default behavior)
- Add PR deep analysis as separate mode (--pr flag or PR URL)
- Dual-mode command handles both use cases

Usage:
  /mgw:review 42           - Review comments on issue #42
  /mgw:review 42 --pr      - Deep PR analysis on PR #42
  /mgw:review https://github.com/owner/repo/pull/42 - Deep PR review

Author: Stephen Miller