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.go

@@ -28,7 +28,7 @@ var commonSurnames = map[string]bool{
 // MatchMembers finds members mentioned in text and returns them with a
 // confidence level of "auto" (reliable) or "review" (needs human verification).
 //
-// Algorithm (ported verbatim from scripts/match_payments.py match_members):
+// Algorithm (ported verbatim from legacy/scripts/match_payments.py match_members):
 //  1. Exact short-circuit: if any member's full normalized name appears as whole
 //     words in normalize(text), return ONLY those matches as auto. This prevents
 //     nickname "tov" from matching inside surname "ottova".