docs: add PR comment design spec from competitive analysis

Based on real competitor PR comments (CodeRabbit, Claude Code Action,
SonarCloud, Snyk, SQLFluff). Includes:
- Detailed analysis of 10+ real PR comments on public repos
- Design principles extracted across all competitors
- 3 full markdown templates (clean, warnings, critical+impact+cost)
- Length guidelines and inline comment strategy

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: anandgupta42