Add YAML tree-sitter language support

[possibilities]

Summary

• Add tree-sitter queries and language mapping for YAML (.yaml, .yml)
• Captures top-level mapping keys only - nested keys are excluded by anchoring query to stream > document > block_node > block_mapping
• Handles both plain and quoted keys (double/single quotes stripped via generalized _strip_wrapping_quotes helper)
• Symbol code spans the full mapping pair so nested values are included in the code field

Testing

pytest tests/test_yaml_symbols.py tests/test_tree_sitter_languages.py tests/test_symbol_extraction_multilang.py -v