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>

legacy/build/entrypoint.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+set -euo pipefail
+
+echo "[entrypoint] Starting gunicorn on port 5001..."
+
+exec gunicorn \
+    --bind 0.0.0.0:5001 \
+    --workers "${GUNICORN_WORKERS:-2}" \
+    --timeout "${GUNICORN_TIMEOUT:-120}" \
+    --access-logfile - \
+    app:app