Add tree-sitter discovery foundation (#125)

[Dimwiddle]

Description

Implement issue #125 discovery-layer foundation for language-aware file discovery:

• Add LanguageRegistry with shared tree-sitter language/parser cache and parse helpers.
• Add FileIndex for repository file indexing, extension/language filtering, and directory pattern queries.
• Add language ratio detection utility (detect_project_languages).
• Add discovery-oriented feature spec stub and parser fixtures.
• Add focused discovery unit tests for registry, index, and language detection.

Type of Change

• Feature

Testing

• make lint
• PYTHONPATH=src pytest tests/discovery/test_file_index.py tests/discovery/test_language_registry.py tests/discovery/test_language_detect.py
• make pre-commit
• PYTHONPATH=src make test (currently fails on 2 existing subprocess import-path cases in acceptance/integration scenarios: generated tests require specleft installed in subprocess execution environment)

Related Issues

Closes #125