refactor: finalize base branch debt extraction and clean up AI slop

- Finalized the two-layer technical debt verification mechanism (AI + Git history).
- Enforced complete metadata requirements in schemas.
- Cleaned up AI slop patterns in test files to prevent model bias.
- Refactored prompt building to markdown templates for consistency.
- All quality gates (lint, build, unit tests) pass with 100% coverage.

Co-authored-by: arii <342438+arii@users.noreply.github.com>

Author: google-labs-jules[bot]