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.go

@@ -1,5 +1,5 @@
 // Package synch ports the bank-sync deduplication helper from
-// scripts/sync_fio_to_sheets.py.
+// legacy/scripts/sync_fio_to_sheets.py.
 package synch
 
 import (
@@ -30,7 +30,7 @@ type Transaction struct {
 // "date|amount|currency|sender|vs|message|bank_id" (lower-cased), used
 // as the dedup key in column K of the payments sheet.
 //
-// Byte-stable with scripts/sync_fio_to_sheets.py generate_sync_id.
+// Byte-stable with legacy/scripts/sync_fio_to_sheets.py generate_sync_id.
 func GenerateSyncID(tx Transaction) string {
 	currency := tx.Currency
 	if currency == "" {