Add non-Markdown rules support

[DandyLyons]

Summary

• keep rules scans Markdown-only by default while adding explicit non-Markdown opt-in
• validate wrapped frontmatter, schemas, JMESPath, type hints, and raw body predicates
• diagnose unsupported Markdown structure on non-Markdown records
• add fixture-based CLI coverage, Core tests, help, and documentation

Verification

• swift build
• swift test (1,107 tests across 93 suites)
• focused RulesNonMDCLISemanticsTests (10 tests)
• DocC warnings-as-errors for md-utils and MarkdownUtilitiesCore

Closes #101