feat(extractor): implement multi-engine article content extractor

- Added scripts/extract_article_contents.py for batch scraping with stealth Foxcape and triple extraction (Trafilatura, Newspaper4k, Readability)
- Created unit, integration, and E2E test suite in tests/test_extract_article_contents.py (90/90 passing)
- Updated specs/003-article-content-extractor and README.md with usage documentation and CLI contracts
- Passed ruff linting/formatting and mypy type checking cleanly

requirements.txt

@@ -3,6 +3,10 @@ foxcape>=0.1.1
 beautifulsoup4>=4.12.0
 googlenewsdecoder>=0.1.7
 selectolax>=0.3.27
+trafilatura>=1.8.0
+newspaper4k>=0.9.3.1
+readability-lxml>=0.8.1
+lxml>=4.9.0
 # Optional Tier 2 / Tier 3 dependencies (not required for POC core execution)
 # sentence-transformers>=2.2.0
 # httpx>=0.24.0