Implement Paratext metadata access

Add load_project_file() and xpath_text() functions for loading
project metadata (Settings.xml, metadata.json). Includes 9 tests,
fixtures, and documentation updates.

- load_project_file(): Generic loader with auto-format detection
- xpath_text(): Helper for extracting text from XML
- 9 tests in tests/test_paratext_metadata.py
- Test fixtures for Settings.xml and metadata.json
- Updated docs/llmflow-language.md, docs/ai-context/*
- Updated example pipeline in tmp/

All 1223 tests passing.

Closes #73
