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/synch/synch_test.go

@@ -6,7 +6,7 @@ import (
 
 // All expected digests verified against the live Python implementation on 2026-05-06:
 //
-//	PYTHONPATH=scripts:. python -c '
+//	PYTHONPATH=legacy/scripts:legacy python -c '
 //	from sync_fio_to_sheets import generate_sync_id
 //	cases = [
 //	    {"date":"2026-01-15","amount":500.0,"currency":"CZK","sender":"Jan Novak","vs":"123","message":"clenske 1/2026","bank_id":"abc123"},
@@ -88,7 +88,7 @@ func TestGenerateSyncID(t *testing.T) {
 
 // All expected strings verified against the live Python implementation on 2026-05-06:
 //
-//	PYTHONPATH=scripts:. python -c '
+//	PYTHONPATH=legacy/scripts:legacy python -c '
 //	for v in [0.0, 500.0, -500.0, 0.1, 1234.56, 99999.99, 1500000.0, 1e16, 1e-5]:
 //	    print(repr(v), "->", repr(str(v)))
 //	'