🪧 add JSON schema and spec for sigmaHQ regression data

[frack113]

Summary

Add a JSON Schema for regression test info.yml files and a complete specification document.

Files Added

• json-schema/sigmahq-regression-data-schema.json — JSON Schema for regression test metadata files
• sigmahq/sigmahq-regression-data.md — Complete specification covering:
  • Directory structure
  • info.yml field requirements
  • Linking regression tests to Sigma rules
  • Test sample file naming convention
  • Test types (currently only Positive Detection Test)
  • Status requirements
  • Validation rules

Files Modified

• specification/sigma-appendix-tags.md — Update MITRE attack URLs (wiki -> new domain)
• specification/sigma-appendix-taxonomy.md — Formatting fix

Related

Closes #204