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>

.gitea/workflows/build.yaml

@@ -31,11 +31,11 @@ jobs:
             TAG=${{ inputs.tag }}
           fi
           IMAGE=gitea.home.hrajfrisbee.cz/${{ github.repository }}:$TAG
-          docker build -f build/Dockerfile \
+          docker build -f legacy/build/Dockerfile \
             --build-arg GIT_TAG=$TAG \
             --build-arg GIT_COMMIT=${{ github.sha }} \
             --build-arg BUILD_DATE=$(date -u +%Y-%m-%dT%H:%M:%SZ) \
-            -t $IMAGE .
+            -t $IMAGE legacy/
           docker push $IMAGE
 
   build-go: