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/reconcile/reconcile.go

@@ -1,4 +1,4 @@
-// Package reconcile ports the three-phase payment reconciliation from scripts/match_payments.py.
+// Package reconcile ports the three-phase payment reconciliation from legacy/scripts/match_payments.py.
 package reconcile
 
 import (
@@ -105,7 +105,7 @@ var questionMarkRe = regexp.MustCompile(`\[\?\]\s*`)
 
 // canonicalMemberKey returns a diacritic-, case-, and whitespace-insensitive key
 // used to resolve Person-column values that drift from canonical attendance-sheet names.
-// Ports scripts/match_payments.py canonical_member_key.
+// Ports legacy/scripts/match_payments.py canonical_member_key.
 func canonicalMemberKey(name string) string {
 	return strings.Join(strings.Fields(czech.Normalize(name)), " ")
 }
@@ -124,7 +124,7 @@ type monthExpected struct {
 // defaultYear seeds czech.ParseMonthReferences in the inference fallback.
 // Pass time.Now().Year() in production; pass a fixed year in tests.
 //
-// Ports scripts/match_payments.py reconcile.
+// Ports legacy/scripts/match_payments.py reconcile.
 func Reconcile(
 	members []Member,
 	sortedMonths []string,

go/internal/domain/reconcile/reconcile_test.go

@@ -1,8 +1,8 @@
 package reconcile
 
-// Expected values verified against scripts/match_payments.py on 2026-05-06:
+// Expected values verified against legacy/scripts/match_payments.py on 2026-05-06:
 //
-//	PYTHONPATH=scripts:. python3 -m unittest tests.test_reconcile_exceptions tests.test_match_payments -v
+//	PYTHONPATH=legacy/scripts:legacy python3 -m unittest tests.test_reconcile_exceptions tests.test_match_payments -v
 //
 // All Python test cases are ported below. Additional Go-only cases are marked with [Go].