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>

.vscode/launch.json

@@ -2,14 +2,16 @@
     "version": "0.2.0",
     "configurations": [
         {
-            "name": "Python Debugger: Flask",
+            "name": "Python Debugger: Flask (legacy)",
             "type": "debugpy",
             "request": "launch",
             "module": "flask",
-            "python": "${workspaceFolder}/.venv/bin/python",
+            "python": "${workspaceFolder}/legacy/.venv/bin/python",
+            "cwd": "${workspaceFolder}/legacy",
             "env": {
                 "FLASK_APP": "app.py",
-                "FLASK_DEBUG": "1"
+                "FLASK_DEBUG": "1",
+                "CREDENTIALS_PATH": "${workspaceFolder}/.secret/fuj-management-bot-credentials.json"
             },
             "args": [
                 "run",