feat: add /speckit.cleanup command for post-implementation review

Add a new cleanup command that runs after implementation to:
- Fix small issues immediately (Scout Rule): debug statements, unused
  imports, dead code, hardcoded localhost URLs
- Create follow-up tasks for medium issues: missing error handling,
  code duplication, incomplete documentation
- Generate tech-debt-report.md for large issues: architectural concerns,
  security vulnerabilities, with options analysis and recommendations

The command includes:
- Constitution compliance checking
- Linter deference for style issues
- User confirmation before applying fixes
- Critical issue detection (halts on hardcoded credentials)
- Idempotent execution (safe to run multiple times)

All outputs stay within the feature directory to maintain traceability
through the full lifecycle: Spec → Plan → Tasks → Implement → Cleanup

Author: dsrednicki