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/io/fio/api.go

@@ -17,7 +17,7 @@ type httpDoer interface {
 }
 
 // apiClient fetches transactions from the Fio REST API (JSON).
-// Ports scripts/fio_utils.py fetch_transactions_api.
+// Ports legacy/scripts/fio_utils.py fetch_transactions_api.
 type apiClient struct {
 	token string
 	hc    httpDoer