feat: add changelog schema for Keep a Changelog validation

Created comprehensive changelog-schema-v1.0.md to validate CHANGELOG.md
files following the Keep a Changelog format. This schema demonstrates
the practical application of the schema evolution system.

**Schema Features**:
- Section validation: Enforces [Unreleased] section presence
- Version format validation: [X.Y.Z] - YYYY-MM-DD pattern
- Semantic versioning compliance
- ISO 8601 date format checking
- Change type subsections: Added, Changed, Deprecated, Removed, Fixed, Security
- Content pattern matching via x-markitect-content-control extensions
- Structural validation via JSON Schema properties

**Validation Results**:
✅ Successfully validates project CHANGELOG.md
✅ All section requirements met (7 sections checked, 11 found)
✅ All content requirements met
✅ All semantic checks passing

**Implementation Notes**:
- H1 "Changelog" title validated via JSON Schema structural checks
- H2 sections validated via x-markitect-sections classifications
- SectionValidator limitation: Only checks H2+ headings, not H1
- Workaround: Structural validation covers H1 title requirement

**Philosophy**: "The release that validates itself"
- v0.10.0 uses its own schema system to validate its CHANGELOG
- Perfect showcase of schema evolution practical value
- Demonstrates x-markitect extensions in real-world use case

**Stage 2 Complete** per release-management-optimization workplan.

Files:
- markitect/schemas/changelog-schema-v1.0.md (new)
- CHANGELOG.md (documented new schema)

CHANGELOG.md

@@ -52,6 +52,14 @@ See history/YYMMDD-ROADMAOTOPIC/ directories for planning information of closed
   - 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
+- **Changelog Schema**: Production schema for validating CHANGELOG.md files following Keep a Changelog format
+  - Schema file: `changelog-schema-v1.0.md` validates version history structure and formatting
+  - Enforces Unreleased section presence (required)
+  - Validates version format: `[X.Y.Z] - YYYY-MM-DD` with semantic versioning
+  - Validates change type subsections: Added, Changed, Deprecated, Removed, Fixed, Security
+  - Content pattern validation for version sections, date formats (ISO 8601), and change types
+  - Demonstrates real-world schema system usage: "The release that validates itself"
+  - Successfully validates project CHANGELOG.md with all semantic checks passing
 
 ### Changed
 - **Directory Reorganization**: