docs: add k8s image GC known issue to debug checklist

Kubernetes image garbage collection silently deletes the nanoclaw-agent
image when disk usage is high because ephemeral containers don't
protect the image from GC. Documents symptoms, cause, fix, and diagnosis.

docs/DEBUG_CHECKLIST.md

@@ -11,6 +11,34 @@ Both timers fire at the same time, so containers always exit via hard SIGKILL (c
 ### 3. Cursor advanced before agent succeeds
 `processGroupMessages` advances `lastAgentTimestamp` before the agent runs. If the container times out, retries find no messages (cursor already past them). Messages are permanently lost on timeout.
 
+### 4. Kubernetes image garbage collection deletes nanoclaw-agent image
+
+**Symptoms**: `Container exited with code 125: pull access denied for nanoclaw-agent` — the container image disappears overnight or after a few hours, even though you just built it.
+
+**Cause**: If your container runtime has Kubernetes enabled (Rancher Desktop enables it by default), the kubelet runs image garbage collection when disk usage exceeds 85%. NanoClaw containers are ephemeral (run and exit), so `nanoclaw-agent:latest` is never protected by a running container. The kubelet sees it as unused and deletes it — often overnight when no messages are being processed. Other images (docker-compose services) survive because they have long-running containers referencing them.
+
+**Fix**: Disable Kubernetes if you don't need it:
+```bash
+# Rancher Desktop
+rdctl set --kubernetes-enabled=false
+
+# Then rebuild the container image
+./container/build.sh
+```
+
+**Diagnosis**: Check the k3s log for image GC activity:
+```bash
+grep -i "nanoclaw" ~/Library/Logs/rancher-desktop/k3s.log
+# Look for: "Removing image to free bytes" with the nanoclaw-agent image ID
+```
+
+Check NanoClaw logs for image status:
+```bash
+grep -E "image found|image NOT found|image missing" logs/nanoclaw.log
+```
+
+If you need Kubernetes enabled, set `CONTAINER_IMAGE` to an image stored in a registry that the kubelet won't GC, or raise the GC thresholds.
+
 ## Quick Status Check
 
 ```bash