refactor: move Python implementation to legacy/, make Go primary

Go (go/) becomes the primary implementation. The original Python
version moves to legacy/ and is kept runnable; new features are still
built in both (see the Dual implementation rule in CLAUDE.md).

- git mv app.py, scripts/, templates/, tests/, pyproject.toml, uv.lock
  and build/ into legacy/
- Makefile: Python targets point at legacy/, venv in legacy/.venv,
  CREDENTIALS_PATH exported for the root .secret/ file
- CI and make image build the Python image with legacy/ as context;
  the Makefile is no longer copied into the image
- legacy/.dockerignore moved to the context root so it takes effect
- CLAUDE.md, docs/README.md, Go comments, fixture README, capture
  script and VS Code launch config updated to the new layout

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

go/internal/domain/matching/match_members_test.go

@@ -1,9 +1,9 @@
 package matching
 
-// Expected values verified against scripts/match_payments.py and
+// Expected values verified against legacy/scripts/match_payments.py and
 // tests/test_match_members.py on 2026-05-06:
 //
-//	PYTHONPATH=scripts:. python3 -c '
+//	PYTHONPATH=legacy/scripts:legacy python3 -c '
 //	from match_payments import match_members
 //	MEMBERS = ["Henrietta Ottová", "Tomáš Němeček (Tov)", "František Vrbík (Štrúdl)", "Jana Nováková"]
 //	cases = [