Add AST-aware code chunking for better code understanding (#58)

* feat(core): Add AST-aware code chunking with astchunk integration

This PR introduces intelligent code chunking that preserves semantic boundaries
(functions, classes, methods) for better code understanding in RAG applications.

Key Features:
- AST-aware chunking for Python, Java, C#, TypeScript files
- Graceful fallback to traditional chunking for unsupported languages
- New specialized code RAG application for repositories
- Enhanced CLI with --use-ast-chunking flag
- Comprehensive test suite with integration tests

Technical Implementation:
- New chunking_utils.py module with enhanced chunking logic
- Extended base RAG framework with AST chunking arguments
- Updated document RAG with --enable-code-chunking flag
- CLI integration with proper error handling and fallback

Benefits:
- Better semantic understanding of code structure
- Improved search quality for code-related queries
- Maintains backward compatibility with existing workflows
- Supports mixed content (code + documentation) seamlessly

Dependencies:
- Added astchunk and tree-sitter parsers to pyproject.toml
- All dependencies are optional - fallback works without them

Testing:
- Comprehensive test suite in test_astchunk_integration.py
- Integration tests with document RAG
- Error handling and edge case coverage

Documentation:
- Updated README.md with AST chunking highlights
- Added ASTCHUNK_INTEGRATION.md with complete guide
- Updated features.md with new capabilities

* Refactored chunk utils

* Remove useless import

* Update README.md

* Update apps/chunking/utils.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update apps/code_rag.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Fix issue

* apply suggestion from @Copilot

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Fixes after pr review

* Fix tests not passing

* Fix linter error for documentation files

* Update .gitignore with unwanted files

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Andy Lee <andylizf@outlook.com>

.gitignore

@@ -93,3 +93,8 @@ packages/leann-backend-diskann/third_party/DiskANN/_deps/
 batchtest.py
 tests/__pytest_cache__/
 tests/__pycache__/
+
+CLAUDE.md
+CLAUDE.local.md
+.claude/*.local.*
+.claude/local/*