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/config/config.go

@@ -42,7 +42,7 @@ const (
 	JuniorSheetGID          = "1213318614" // gid=1213318614 — junior practices tab (IMPORTRANGE'd)
 )
 
-// CacheSheetMap mirrors scripts/config.py CACHE_SHEET_MAP.
+// CacheSheetMap mirrors legacy/scripts/config.py CACHE_SHEET_MAP.
 // Maps a cache key to the Google Sheet ID whose Drive modifiedTime gates it.
 // Both attendance keys map to the same spreadsheet — different tabs, one Drive file.
 var CacheSheetMap = map[string]string{
@@ -53,7 +53,7 @@ var CacheSheetMap = map[string]string{
 }
 
 // Config holds all runtime configuration loaded from environment variables.
-// Mirrors scripts/config.py.
+// Mirrors legacy/scripts/config.py.
 type Config struct {
 	CredentialsPath  string
 	QRAccount        string          // IBAN of the primary account used for QR codes