feat: implement Phase 2 - Markdown Schema Loader

Completed Phase 2 of the schema-of-schemas implementation with full
markdown schema support. This enables schemas to be authored as
markdown files with rich documentation and embedded JSON schemas.

Core Implementation (markitect/schema_loader.py):
- MarkdownSchemaLoader class with comprehensive parsing capabilities
- YAML frontmatter extraction with error handling
- JSON code block extraction with section preference (## Schema Definition)
- Metadata merging with x-markitect-source tracking
- Schema saving with template support and round-trip capability
- Helper methods: list_json_blocks(), validate_schema_structure()

Test Coverage (tests/test_schema_loader.py):
- 35 comprehensive unit tests (100% passing)
- Tests for loading, parsing, saving, round-trip conversion
- Edge case handling (empty files, binary files, malformed blocks)
- Fixed binary file test to use invalid UTF-8 sequences

Example Schema (markitect/schemas/manpage-schema-v1.0.md):
- First markdown schema following naming convention
- Complete manpage schema with frontmatter + documentation + JSON
- Demonstrates section classification and content control
- Shows proper structure for future schema authors

Documentation (roadmap/schema-of-schemas/SCHEMA_LOADER_GUIDE.md):
- Comprehensive user guide (600+ lines)
- API reference with examples
- Best practices and troubleshooting
- Integration patterns for CLI and validator

Progress Tracking:
- Updated TODO.md with Phase 2 completion
- Updated CHANGELOG.md with implementation details
- Next: Phase 3 - Schema-for-Schemas Metaschema

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

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

CHANGELOG.md

@@ -35,11 +35,13 @@ 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 1 of 6)
-  - Implementing filename validation for schema naming convention
-  - Building markdown schema loader to parse `.md` schema files
-  - Creating schema-for-schemas metaschema for schema validation
-  - Planning migration of 5 existing schemas to new format (will remove 2 duplicates)
+- **Schema-of-Schemas Implementation** (Phase 2 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 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
 
 ## [0.9.0] - 2025-11-14