feat(docker): add isolated Claude Code environment with proxy sidecar

Two-container setup: claude (UID 1000, internal-only network) and proxy
(Squid, UID 13). The internal Docker network uses internal: true so the
claude container has no direct internet route. All egress is tunnelled
through the Squid sidecar which enforces a domain allowlist. Both
containers drop all capabilities and set no-new-privileges. claude.sh
provides start/stop/run/update/logs/status/shell lifecycle management.

Dockerfile.proxy

@@ -0,0 +1,25 @@
+FROM ubuntu:22.04
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+        squid \
+    && rm -rf /var/lib/apt/lists/*
+
+# Give the proxy system user (UID 13) ownership of all Squid paths
+RUN mkdir -p /var/spool/squid /var/log/squid \
+    && chown -R proxy:proxy /var/spool/squid /var/log/squid /etc/squid
+
+COPY --chown=proxy:proxy proxy/squid.conf /etc/squid/squid.conf
+
+USER proxy
+
+# Initialise cache directories as the proxy user
+RUN squid -N -f /etc/squid/squid.conf -z 2>/dev/null || true
+
+EXPOSE 3128
+
+HEALTHCHECK --interval=10s --timeout=5s --retries=3 \
+    CMD /bin/bash -c 'echo >/dev/tcp/127.0.0.1/3128'
+
+CMD ["squid", "-N", "-f", "/etc/squid/squid.conf"]