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/uc12_lead_list_handoff.csv

@@ -0,0 +1,15 @@
+lead_id,first_name,last_name,email,phone,company,lead_source,captured_date
+L001,Patricia,Anders,p.anders@bigco.com,(555) 100-2222,Big Co,trade_show,2026-02-15
+L045,Pat,Anders,p.anders@bigco.com,5551002222,Big Co Inc,linkedin,2026-03-01
+L091,Patty,Anders,panders@bigco.com,(555) 100-2222,BigCo,webinar,2026-03-22
+L010,Mohammed,Khan,m.khan@example.com,(555) 200-3333,Khan Industries,trade_show,2026-02-15
+L032,Muhammad,Khan,mkhan@example.com,(555) 200-3333,Khan Industries,cold_outreach,2026-02-28
+L067,Mohammad,Khan,m.khan@example.com,(555) 200-3333,Khan Industries Inc,referral,2026-03-10
+L020,Sarah,"Lee, VP Marketing",slee@target.com,(555) 300-4444,Target Corp,linkedin,2026-02-20
+L058,Sarah,Lee,slee@target.com,(555) 300-4444,Target Corp,webinar,2026-03-05
+L080,John,Adams,jadams@enterprise.com,(555) 400-5555,Enterprise Inc,trade_show,2026-03-15
+L081,John,Adams II,jadams2@enterprise.com,(555) 400-5556,Enterprise Inc,trade_show,2026-03-15
+L100,Wei,Chen,wchen@startup.io,(555) 500-6666,Startup IO,referral,2026-03-20
+L101,Olivia,Mendez,omendez@agency.com,(555) 600-7777,Agency Co,linkedin,2026-03-21
+L102,Raj,Patel,rpatel@firm.com,(555) 700-8888,Firm LLC,cold_outreach,2026-03-22
+L103,Emma,Wright,ewright@consulting.com,(555) 800-9999,Wright Consulting,webinar,2026-03-23