feat: implement Phase 3 - Schema-for-Schemas Metaschema

Completed Phase 3 of the schema-of-schemas implementation with a
comprehensive metaschema that validates all MarkiTect schema files
against conventions and standards.

Metaschema Implementation (schema-schema-v1.0.md - 650+ lines):
- Validates core JSON Schema fields ($schema, $id, title, description)
- Validates MarkiTect version field (SemVer: major.minor.patch)
- Validates $id URL format (HTTPS with version path)
- Validates MarkiTect extensions:
  - x-markitect-sections: section classifications and content rules
  - x-markitect-content-control: pattern and quality validation
  - x-markitect-metadata: status, authors, tags
  - x-markitect-source: loader metadata (auto-added)
- Section classification validation (required, recommended, optional,
  discouraged, improper)
- Content control pattern validation
- Comprehensive documentation with examples and usage guides

CLI Command (markitect schema-validate):
- Validates schema files against metaschema
- Supports both markdown and JSON schema files
- Detailed error reporting with schema paths
- Structure validation recommendations
- Exit codes for CI/CD integration

Test Coverage (tests/test_schema_metaschema.py - 12 tests, 100% passing):
- Metaschema self-validation
- Manpage schema validation
- Required fields enforcement
- Version format validation (valid and invalid cases)
- $id format validation (valid and invalid cases)
- Section classification validation
- Complete schema with all extensions

Validation Results:
- ✅ Metaschema validates itself successfully
- ✅ Manpage schema (v1.0.md) validates successfully
- ⚠️  Terminology schema needs migration (missing version, incorrect $id)

Progress Tracking:
- Updated TODO.md with Phase 3 completion
- Updated CHANGELOG.md with implementation details
- Next: Phase 4 - Schema Migration

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

CHANGELOG.md

@@ -35,10 +35,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - **BREAKING**: Legacy DocumentControls component from TestDrive JSUI plugin system - all control panel functionality now provided by enhanced control panels (ContentsControl, StatusControl, DebugControl, EditControl) with Reset All button functionality moved to EditControl for better maintainability and elimination of code duplication
 
 ### In Progress
-- **Schema-of-Schemas Implementation** (Phase 2 of 6 - Completed ✅)
+- **Schema-of-Schemas Implementation** (Phase 3 of 6 - Completed ✅)
   - ✅ Phase 1: Filename validation for schema naming convention (`markitect/schema_naming.py`, 50 tests)
   - ✅ Phase 2: Markdown schema loader to parse `.md` schema files (`markitect/schema_loader.py`, 35 tests)
-  - ⏳ Phase 3: Creating schema-for-schemas metaschema for schema validation
+  - ✅ Phase 3: Schema-for-schemas metaschema for schema validation (`schema-schema-v1.0.md`, 12 tests)
   - ⏳ Phase 4: Migration of 5 existing schemas to new format (will remove 2 duplicates)
   - ⏳ Phase 5: CLI updates and documentation
   - ⏳ Phase 6: Integration testing and validation