fix: vision(#623): disinto-chat sandbox hardening (#706)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/chat/entrypoint-chat.sh

@@ -12,6 +12,16 @@ log() {
     printf '[%s] %s\n' "$(date -u '+%Y-%m-%d %H:%M:%S UTC')" "$*" | tee -a "$LOGFILE"
 }
 
+# Sandbox sanity checks (#706) — fail fast if isolation is broken
+if [ -e /var/run/docker.sock ]; then
+    log "FATAL: /var/run/docker.sock is accessible — sandbox violation"
+    exit 1
+fi
+if [ "$(id -u)" = "0" ]; then
+    log "FATAL: running as root (uid 0) — sandbox violation"
+    exit 1
+fi
+
 # Verify Claude CLI is available (expected via volume mount from host).
 if ! command -v claude &>/dev/null; then
     log "FATAL: claude CLI not found in PATH"