Replace the 1-line placeholder with a proper Hub README:
image variants table, quick start (docker run + docker compose),
inherited-from-base + added-by-pi-devbox feature lists, versioning
scheme, persistent volumes table, user-installed pi packages note,
source links.
Already PATCH'd live on Docker Hub manually — this commit keeps the
in-repo file in sync so the next tag-triggered update-description job
won't roll it back to the stub.
pi coding-agent container built on opencode-devbox:base-latest.
Includes Dockerfile, docker-compose, CI workflow, smoke-test,
README, CHANGELOG, AGENTS.md.
joakimp