ci: add Gitea Actions workflow and Docker Compose test runner; add test image\n\ngovernance: encode CI/containers rules in system prompts and AGENTS templates; update proposal and docs

docker/test/Dockerfile

@@ -0,0 +1,11 @@
+FROM debian:bookworm-slim
+
+ENV DEBIAN_FRONTEND=noninteractive
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends \
+       ca-certificates git bash curl jq yq \
+       bats \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /work
+