docs: add /model allowlist troubleshooting note

docs/help/troubleshooting.md

@@ -53,6 +53,15 @@ Almost always a Node/npm PATH issue. Start here:
 - [Models](/cli/models)
 - [OAuth / auth concepts](/concepts/oauth)
 
+### `/model` says `model not allowed`
+
+This usually means `agents.defaults.models` is configured as an allowlist. When it’s non-empty,
+only those provider/model keys can be selected.
+
+- Check the allowlist: `clawdbot config get agents.defaults.models`
+- Add the model you want (or clear the allowlist) and retry `/model`
+- Use `/models` to browse the allowed providers/models
+
 ### When filing an issue
 
 Paste a safe report: