feat: implement tree-sitter AST parser for Python (Phase 1 Step 2)

[vineethwilson15]

Goal

Implement the Python AST parser in core/ast_parsers/ using tree-sitter.

Acceptance Criteria

• PythonParser class with parse(path, source) -> list[dict] interface
• Extracts name, kind, start_line, end_line, docstring
• Returns empty list for files with syntax errors (never raises)
• Unit tests in tests/unit/test_python_parser.py with mocked tree-sitter
• Indexer writes each symbol as a Symbol node in Neo4j with a DEFINES rel to the parent File