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>

test-cases/uc05_subscriber_list.csv

@@ -0,0 +1,16 @@
+email,first_name,source,subscribed_date,tags
+sub1@example.com,Pat,newsletter_signup,2025-08-15,newsletter
+SUB1@example.com,Pat,facebook_lead_form,2025-09-01,fb_q3_2025
+sub1@EXAMPLE.com,Patricia,checkout_optin,2025-10-12,customer
+sub1@example.com ,Pat W,popup_form,2026-01-08,popup_homepage
+mike.smith@gmail.com,Mike,import_2024,2024-11-20,legacy
+mikesmith@gmail.com,Mike S,newsletter_signup,2025-03-15,newsletter
+m.i.k.e.s.m.i.t.h@gmail.com,Michael,facebook_lead_form,2025-07-22,fb_q3_2025
+promos+freebie@gmail.com,Sam,freebie_download,2025-12-01,lead_magnet
+promos@gmail.com,Sam,newsletter_signup,2026-02-15,newsletter
+unique1@example.com,Alpha,newsletter_signup,2025-09-10,newsletter
+unique2@example.com,Beta,popup_form,2025-10-05,popup_homepage
+unique3@example.com,Gamma,facebook_lead_form,2025-11-12,fb_q4_2025
+unique4@example.com,Delta,checkout_optin,2026-01-20,customer
+jdoe@company-a.com,Jane Doe,newsletter_signup,2025-08-01,newsletter
+jdoe@company-b.com,Jane Doe,newsletter_signup,2025-08-02,newsletter