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historical reference.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>

TODO.md

@@ -14,8 +14,8 @@ This section is for tasks currently being discussed with or worked on by the cod
 
 ### Schema-of-Schemas Implementation (Active - Phase 4)
 
-**Status:** Phase 4 - Schema Migration (Completed ✅)
-**Workplan:** See `roadmap/schema-of-schemas/WORKPLAN.md`
+**Status:** Completed (All 6 Phases ✅)
+**Workplan:** See `history/2026-01-05-schema-of-schemas/WORKPLAN.md` (archived)
 
 **Current Goals:**
 1. ✅ Establish naming convention: `{domain}-schema-v{major}.{minor}.md`