Remove old docker-compose based dev setup

The devcontainer replaces Dockerfile.dev, dc.dev.yml and the related
start/entrypoint scripts.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.devcontainer/Dockerfile

@@ -1,4 +1,4 @@
-# Keep the Python version in sync with Dockerfile.dev
+# Keep the Python version in sync with the production Dockerfile
 FROM python:3.15-slim-trixie
 
 ARG USERNAME=dev