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>

TODO.md

@@ -12,9 +12,9 @@ The structure organizes **future tasks** by their impact, just as a changelog or
 
 This section is for tasks currently being discussed with or worked on by the coding assistant. These are the ephemeral, flow-of-thought tasks.
 
-### Schema-of-Schemas Implementation (Active - Phase 2)
+### Schema-of-Schemas Implementation (Active - Phase 3)
 
-**Status:** Phase 2 - Markdown Schema Loader (Completed ✅)
+**Status:** Phase 3 - Schema-for-Schemas Metaschema (Completed ✅)
 **Workplan:** See `roadmap/schema-of-schemas/WORKPLAN.md`
 
 **Current Goals:**
@@ -22,8 +22,8 @@ This section is for tasks currently being discussed with or worked on by the cod
 2. ✅ Implement filename validation logic
 3. ✅ Create markdown schema loader
 4. ✅ Create example markdown schema
-5. ⏳ Build schema-for-schemas metaschema (Next: Phase 3)
-6. ⏳ Migrate existing schemas to new format
+5. ✅ Build schema-for-schemas metaschema
+6. ⏳ Migrate existing schemas to new format (Next: Phase 4)
 
 **Phase 1 Tasks (Completed ✅):**
 - [x] Write `markitect/schema_naming.py` with validation logic
@@ -39,13 +39,20 @@ This section is for tasks currently being discussed with or worked on by the cod
 - [x] Create example markdown schema (manpage-schema-v1.0.md)
 - [x] Create SCHEMA_LOADER_GUIDE.md documentation
 
+**Phase 3 Tasks (Completed ✅):**
+- [x] Design schema-for-schemas metaschema (schema-schema-v1.0.md)
+- [x] Implement metaschema with validation rules for MarkiTect conventions
+- [x] Add schema-validate CLI command with detailed error reporting
+- [x] Write comprehensive unit tests (12 tests, 100% passing)
+- [x] Test metaschema self-validation
+- [x] Validate existing schemas against metaschema
+
 **Next Phases:**
-- Phase 3: Schema-for-Schemas Metaschema (2 days)
 - Phase 4: Schema Migration (1-2 days)
 - Phase 5: CLI & Documentation Updates (1 day)
 - Phase 6: Testing & Validation (1 day)
 
-**Expected Completion:** 6-7 days remaining
+**Expected Completion:** 4-5 days remaining
 
 ---
 
@@ -163,6 +170,30 @@ The **capability-capability** includes:
 - Shows section classification and content control
 - Follows naming convention: {domain}-schema-v{major}.{minor}.md
 
+### 2026-01-04 - Phase 3: Schema-for-Schemas Metaschema
+- ✅ Created schema-schema-v1.0.md metaschema (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)
+- ✅ Validates MarkiTect extensions (x-markitect-sections, x-markitect-content-control, x-markitect-metadata)
+- ✅ Implemented schema-validate CLI command with detailed error reporting
+- ✅ Comprehensive test suite (12 unit tests, 100% passing)
+- ✅ Metaschema self-validation successful
+
+**Key Features Delivered:**
+- Complete metaschema for validating all MarkiTect schemas
+- Section classification validation (required, recommended, optional, discouraged, improper)
+- Content control pattern validation
+- Version format enforcement (SemVer)
+- $id URL format enforcement (HTTPS with version)
+- CLI command for easy schema validation
+- Detailed error messages with schema paths
+
+**Validation Results:**
+- ✅ Metaschema validates itself
+- ✅ Manpage schema validates successfully
+- ⚠️  Terminology schema needs migration (missing version field, incorrect $id format)
+
 ### 2025-12-17 - Architecture Refactoring
 - ✅ Implemented ReusableCapabilitiesArchitecture v0.1
 - ✅ Added feedback capability to issue-facade