Add source-controlled LOC ratchet baseline

[stef-k]

Summary

• add an explicit source-controlled LOC ratchet for adopting established repositories with legacy oversized files
• support deterministic baseline creation and non-increasing lowering/pruning
• preserve zero-baseline behavior for new projects and keep ordinary analysis read-only
• document lifecycle, precedence, safety, and the distinction from exclusions and exemptions

Validation

• \python -m unittest tests.test_loc_baseline\ (3 passed)
• \python -m unittest discover -s tests\ (297 passed)
• \git diff --check main...HEAD\
• zero-baseline whole-repository audit completed with REVIEW and no FAIL

Agent Code Guard itself does not create or commit a LOC baseline.

Closes #93