docs: add anthropic auth error troubleshooting

docs/gateway/troubleshooting.md

@@ -31,6 +31,24 @@ See also: [Health checks](/gateway/health) and [Logging](/logging).
 
 ## Common Issues
 
+### No API key found for provider "anthropic"
+
+This means the **agent’s auth store is empty** or missing Anthropic credentials.
+Auth is **per agent**, so a new agent won’t inherit the main agent’s keys.
+
+Fix options:
+- Re-run onboarding and choose **Anthropic** for that agent.
+- Or paste a setup-token on the **gateway host**:
+  ```bash
+  clawdbot models auth setup-token --provider anthropic
+  ```
+- Or copy `auth-profiles.json` from the main agent dir to the new agent dir.
+
+Verify:
+```bash
+clawdbot models status
+```
+
 ### OAuth token refresh failed (Anthropic Claude subscription)
 
 This means the stored Anthropic OAuth token expired and the refresh failed.