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/name_variants.go

@@ -19,7 +19,7 @@ var (
 // this invariant for the exact short-circuit pass. Variants shorter than 3 characters are
 // dropped.
 //
-// Ports scripts/match_payments.py _build_name_variants.
+// Ports legacy/scripts/match_payments.py _build_name_variants.
 func BuildNameVariants(name string) []string {
 	var nickname string
 	if m := nicknameRe.FindStringSubmatch(name); m != nil {