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/money/money.go

@@ -1,4 +1,4 @@
-// Package money ports Czech-locale currency parsing from scripts/infer_payments.py.
+// Package money ports Czech-locale currency parsing from legacy/scripts/infer_payments.py.
 package money
 
 import (
@@ -11,7 +11,7 @@ import (
 var ErrInvalidAmount = errors.New("money: invalid CZK amount")
 
 // ParseCZK parses a Czech-locale amount string and returns the value in CZK
-// as a float64. Mirrors scripts/infer_payments.py parse_czk_amount:
+// as a float64. Mirrors legacy/scripts/infer_payments.py parse_czk_amount:
 //
 //   - empty input → (0, nil)
 //   - "Kč"/"CZK" suffixes stripped (case-sensitive, like Python)