Add fixture-backed Sigma backend validation gates

[Errordog2]

/claim #2221

Summary

• Adds a fixture-backed backend validation gate before detection coverage can be marked Tested or Operational.
• Requires TP and benign fixtures, saved backend conversion output, expected/actual alert counts, backend parity review, and regression replay evidence.
• Updates coverage definitions, detection-as-code CI/CD stages, output template, common pitfalls, and changelog for detection-engineering v1.0.1.

Why

Issue #2221 notes that synthetic validation was treated as sufficient even when Sigma rules lacked replayable fixture evidence and backend-specific conversion checks. This change makes the promotion criteria explicit and prevents Sigma-authored-only or Atomic-only coverage from being mislabeled as Tested.

Validation

• git diff --check
• Frontmatter required-field check across skills and roles
• index.yaml file existence check
• Prompt-injection phrase scan across skills and roles
• Targeted rg check for version and new validation sections