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/tests/fixtures/README.md

@@ -85,10 +85,10 @@ git diff go/tests/fixtures/   # review changes before committing
 To refresh a single function:
 
 ```bash
-PYTHONPATH=scripts:. python3 scripts/capture_fixtures.py --func normalize --all \
+PYTHONPATH=legacy/scripts:legacy python3 legacy/scripts/capture_fixtures.py --func normalize --all \
   | while IFS= read -r line; do
       id=$(echo "$line" | python3 -c "import sys,json; print(json.load(sys.stdin)['case'])")
-      echo "$line" | python3 scripts/scrub_fixtures.py \
+      echo "$line" | python3 legacy/scripts/scrub_fixtures.py \
         > go/tests/fixtures/pure/normalize/${id}.json
     done
 ```
@@ -120,7 +120,7 @@ verify that no real roster names appear in their `member_names` arrays.
 
 ## Adding a new fixture
 
-1. Add a seed to `scripts/_fixture_seeds.py` under `SEEDS[("func_name", "case_id")]`.
+1. Add a seed to `legacy/scripts/_fixture_seeds.py` under `SEEDS[("func_name", "case_id")]`.
 2. Add `In`/`Out` struct fields to `go/tests/parity/parityio.go` if the function
    is new.
 3. Run the single-file capture recipe above and review the diff.