Feature: change safety testability refactors

[alxxjohn]

Summary

This branch expands CodeGuard from static code-quality scanning into PR-level change-safety analysis. It adds a new Change Safety check family with config/profile support, detector coverage, PR-summary metrics, local quality precision rules, and
user-facing documentation.

What changed

• Added a new checks.change configuration section and ChangeRulesConfig.
• Added profile behavior:
  • startup keeps change-safety off by default
  • strict and enterprise enable change-safety
  • AI-safe applies tighter change-size/test-ratio thresholds
• Added rule catalog metadata and fix templates for:
  • change.*
  • testing.*
  • refactor.*
  • naming/function/error/defensive/maintainability precision rules
• Added a diff-mode Change Safety runner gated by cfg.Checks.Change.
• Added change concentration detectors for:
  • oversized diffs
  • mixed concerns
  • too many concern families
  • mixed refactor and behavior changes
  • unnecessary public-surface expansion
  • moves without verification
• Added testability detectors for:
  • behavior changes without tests
  • missing failure-path tests
  • hardwired dependencies in business logic
  • nondeterministic domain logic
• Added local quality precision rules for:
  • ambiguous/generic names
  • function responsibility issues
  • weak error handling
  • defensive-programming gaps
  • maintainability regressions
• Added additive pr_summary metrics for:
  • change_safety
  • maintainability_delta
  • refactor_confidence
• Updated docs and examples with the new change-safety configuration and check glossary.

Testing

Verified with targeted detector, PR-summary, metadata/docs, and broad internal package test gates.

Notes

• The new change/testability checks are intentionally diff-first.
• Full scans no-op for the Change Safety section unless future rules add safe repo-level diagnostics.
• Refactor confidence is heuristic/evidence-based; this branch does not claim semantic equivalence proof.