chore: archive completed schema-of-schemas implementation

Moved schema-of-schemas planning artifacts from roadmap to history
with datestamp prefix, marking completion of all 6 implementation phases.

**Changes:**
- Moved roadmap/schema-of-schemas/ → history/2026-01-05-schema-of-schemas/
- Updated all documentation references to new location
- Marked implementation as completed in TODO.md
- Updated CHANGELOG.md to reflect archived status

**Implementation Summary:**
All 6 phases completed successfully:
- Phase 1: Filename validation (50 tests)
- Phase 2: Markdown schema loader (35 tests)
- Phase 3: Schema-for-schemas metaschema (12 tests)
- Phase 4: Schema migration (2 migrated, 3 deleted)
- Phase 5: CLI enhancements (multi-schema validation)
- Phase 6: Integration testing and documentation

**Deliverables:**
- 97 unit tests (100% passing)
- 4 production schemas in registry
- Comprehensive user documentation
- Updated examples (manpages, terminology)
- Complete schema management system

The schema-of-schemas topic is now complete and archived for
historical reference.

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

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

docs/SCHEMA_MANAGEMENT_GUIDE.md

@@ -387,9 +387,10 @@ Planned features:
 
 ## Related Documentation
 
-- [Schema Naming Specification](../roadmap/schema-of-schemas/SCHEMA_NAMING_SPEC.md)
-- [Schema Loader Guide](../roadmap/schema-of-schemas/SCHEMA_LOADER_GUIDE.md)
+- [Schema Naming Specification](../history/2026-01-05-schema-of-schemas/SCHEMA_NAMING_SPEC.md)
+- [Schema Loader Guide](../history/2026-01-05-schema-of-schemas/SCHEMA_LOADER_GUIDE.md)
 - [Metaschema Reference](../markitect/schemas/schema-schema-v1.0.md)
+- [Implementation Workplan](../history/2026-01-05-schema-of-schemas/WORKPLAN.md) (archived)
 
 ## Support