refactor: FROM opencode-devbox:latest-pi-only (lean, no opencode)

Re-point the re-brand at the new pi-only variant instead of with-pi, so
pi-devbox stays a lean pi-focused image (no opencode) while the pi install
logic still lives in one place upstream. This keeps pi-devbox meaningfully
distinct from opencode-devbox:latest-with-pi.

- Dockerfile: BASE_IMAGE default -> joakimp/opencode-devbox:latest-pi-only.
- smoke-test.sh: size threshold 2900 -> 2750 MB (pi-only = with-pi minus
  opencode's ~145 MB binary).
- Docs (README/AGENTS/DOCKER_HUB/CHANGELOG/docker-compose): drop the
  'also contains opencode' notes; describe pi-only basis and the distinction
  from with-pi.

Publish ordering unchanged: release opencode-devbox first so latest-pi-only
carries the target pi version, then tag here (smoke asserts pi --version).

docker-compose.yml

@@ -17,8 +17,8 @@ services:
     # build:
     #   context: .
     #   args:
-    #     # Pin a specific with-pi build instead of tracking latest-with-pi:
-    #     BASE_IMAGE: "joakimp/opencode-devbox:v1.15.13-with-pi"
+    #     # Pin a specific pi-only build instead of tracking latest-pi-only:
+    #     BASE_IMAGE: "joakimp/opencode-devbox:v1.15.13-pi-only"
     container_name: pi-devbox
     stdin_open: true
     tty: true