feat: Complete Issue #2 - Fast Document Loading & CLI Manipulation

Major milestone: Implemented complete document manipulation workflow with
roundtrip validation capabilities.

New features:
- markitect get: Retrieve and output processed markdown files
- markitect modify: Content manipulation with --add-section and --update-front-matter
- AST serialization: Complete AST-to-Markdown conversion with modification support
- Roundtrip validation: add → modify → get → verify workflow operational

Implementation details:
- Added markitect/serializer.py with comprehensive AST-to-Markdown serialization
- Extended CLI with get and modify commands using Click framework
- Support for section addition and front matter updates
- Comprehensive error handling and user feedback
- Integration with existing AST cache and database systems

Testing:
- All 11 Issue #2 tests passing (100% success rate)
- Manual roundtrip validation successfully completed
- Performance optimization maintained (<50% cache loading time)
- Core USP 'Parse once, manipulate many times' fully operational

Files changed:
- NEW: markitect/serializer.py (AST serialization and modification)
- MODIFIED: markitect/cli.py (added get and modify commands)
- Test files demonstrating working roundtrip functionality

Issue #2 requirements fully satisfied:
✅ Performance-first storage strategy
✅ Complete CLI workflow with roundtrip validation
✅ Document manipulation capabilities
✅ AST serialization and content modification
✅ All success criteria met

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

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

test_frontmatter.md

@@ -0,0 +1,8 @@
+---
+title: Test Document
+status: draft
+---
+
+# Test with Front Matter
+
+This document has YAML front matter.