chore: baseline container POC and atomic foundation plan

- Containerized Pi hello-world proof (image mosaic-poc-agent:0.84.4, non-root)
- Four immutable contract fixtures loaded into a generated system prompt
- build/hello/verify/reset scripts with exact-match gating and reset safety
- Documented Pi discovery (v0.84.4, -p mode, --system-prompt, container auth)
- Append-only BUILD-LOG with corrections; deferred layers in LAYERS.md
- Architecture plan: docs/plans/2026-09-02_atomic-mosaic-foundation.md

Containerfile

@@ -0,0 +1,40 @@
+# Minimal Mosaic Stack POC agent image.
+# Base: maintained Node.js image (same family as Pi's documented
+# containerization example in docs/containerization.md).
+FROM node:24-bookworm-slim
+
+# Tools Pi's documented container image expects (bash, CA certs, git, ripgrep).
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends bash ca-certificates git ripgrep \
+    && rm -rf /var/lib/apt/lists/*
+
+# Non-root user: the maintained node image ships a 'node' user at
+# uid/gid 1000, which matches the host user that owns the runtime
+# state directory mounted at /var/lib/mosaic. It is reused as-is.
+
+# Pinned Pi install: package.json pins the exact version and
+# package-lock.json is installed with npm ci. No unversioned installs.
+WORKDIR /opt/app
+COPY package.json package-lock.json ./
+RUN npm ci --ignore-scripts
+
+# Immutable contract fixtures (required location) and runtime scripts.
+COPY contracts /opt/mosaic/contracts
+COPY src /opt/mosaic/src
+RUN chmod 0555 /opt/mosaic/contracts /opt/mosaic/contracts/* \
+    && chmod 0555 /opt/mosaic/src /opt/mosaic/src/*.sh
+
+# Writable state, workspace, and pi agent directory (auth.json is
+# bind-mounted read-only at runtime; nothing is copied into the image).
+RUN mkdir -p /var/lib/mosaic /workspace /home/node/.pi/agent \
+    && chown -R node:node /var/lib/mosaic /workspace /home/node /opt/app
+
+USER node
+WORKDIR /workspace
+ENV HOME=/home/node \
+    PATH="/opt/app/node_modules/.bin:${PATH}" \
+    PI_OFFLINE=1
+
+# One-shot agent: args form the user request (default is the startup
+# verification request defined in compose.yaml).
+ENTRYPOINT ["/opt/mosaic/src/run-agent.sh"]