feat: add documentation, Streamlit GUI, and full source tree

- Rewrite README.md with project overview, quick-start, and CLI summary
- Add docs/CLI-REFERENCE.md with full flag reference and 8 recipe sections
- Add docs/DEVELOPER.md with architecture, data flow, and extension guides
- Rewrite src/core/__init__.py with public API exports and module docstring
- Add Streamlit GUI (src/gui/) with file upload, advanced options, interactive
  match group review with side-by-side diff, and download buttons
- Add .gitignore, requirements.txt, all source code, tests, and sample data
- Add streamlit to requirements.txt

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

requirements.txt

@@ -0,0 +1,10 @@
+pandas>=2.2,<3
+openpyxl>=3.1,<4
+numpy>=1.26,<3
+rapidfuzz>=3.6,<4
+charset-normalizer>=3.3,<4
+loguru>=0.7,<1
+tqdm>=4.66,<5
+typer>=0.12,<1
+phonenumbers>=8.13,<9
+streamlit>=1.35,<2