docs: update CHANGELOG with LinkValidator feature

Added link validation details to semantic validation entry:
- Internal link validation (fragments and file paths) by default
- External link validation with --check-links flag (opt-in)
- Email validation for mailto: links
- Updated test coverage: 25 tests (16 section/content + 9 link)

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Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -39,10 +39,15 @@ See history/YYMMDD-ROADMAOTOPIC/ directories for planning information of closed
   - Section classification enforcement: required/recommended/optional/discouraged/improper sections validated
   - Content pattern validation: required_patterns, forbidden_patterns, discouraged_patterns with regex matching
   - Quality metrics checking: min_words, max_words, min_sentences validation with configurable thresholds
-  - Modular validator architecture: SectionValidator, ContentValidator with clean separation of concerns
+  - Link validation: Internal/external link checking with configurable policies
+    - Internal links: Fragment anchors (#section) and file paths validated by default
+    - External links: HTTP/HTTPS validation with --check-links flag (opt-in, may be slow)
+    - Email validation: mailto: link format checking
+    - Broken link detection with line numbers and detailed error messages
+  - Modular validator architecture: SectionValidator, ContentValidator, LinkValidator with clean separation of concerns
   - CLI integration: `--semantic/--no-semantic`, `--strict`, `--check-links` flags for validate command
   - Comprehensive reporting: Detailed validation reports with errors/warnings, line numbers, matched text
-  - Test coverage: 16 tests for semantic validators, 100% passing
+  - Test coverage: 25 tests for semantic validators (16 section/content + 9 link), 100% passing
   - Full documentation: Semantic validation guide in SCHEMA_MANAGEMENT_GUIDE.md with examples
   - Complements existing structural AST validation for complete document compliance checking