Adopt Code Guard and retire the repository LOC checker

[stef-k]

Replaces the repository LOC checker with published Agent Code Guard 0.3.1. Existing large source files retain their current sizes through 99 non-increasing LOC allowances; generated/vendor boundaries and all six shipped guards are preserved.

Closes #523

CI installs Code Guard in an isolated Python environment and checks the exact PR base SHA, including documentation-only PRs. The native implementation, old baseline, tests, solution/project references, and agent instructions are removed. Testing guidance documents installation, skill activation, exits, ratchet maintenance, and exclusions.

Validation:

• Version and doctor: 0.3.1, healthy; active global skill matches the installed payload.
• Both gates passed in initial CI run 33916319054 before native-tool deletion.
• All 99 allowances independently match current nonblank counts; no recorded old allowance increased.
• Temporary legacy-growth and new-601-line probes both failed as expected, preserving baseline bytes. Probes removed.
• Final full audit: 1,035 analyzed files, REVIEW only, zero FAIL/INCOMPLETE. Baseline unchanged.
• Complete changed scope: REVIEW only for cumulative changelog size and the established testing reference section; accepted as cohesive documentation.
• Build: zero warnings/errors; remaining Tools test: 1 passed; diff check passed.
• Generic global agent instructions updated separately; no project-specific exclusions/allowances copied globally.

No application behavior or database migration changes. Final PR CI remains the merge gate.