docs(skill): guide agent to prompt for --categoryExtensions (#2189)

...when extension tools are missing. I've run into the situation that
the coding agent would try different esoteric approach to install the
extension when asked to debug it in chrome DevTools.

skills/chrome-devtools/SKILL.md

@@ -41,6 +41,21 @@ You can send multiple tool calls in parallel, but maintain correct order: naviga
 
 ### Testing an extension
 
+> **Before proceeding**: Extension tools (`install_extension`, `list_extensions`, etc.) are only available when the MCP server is started with the `--categoryExtensions` flag. If these tools are not in your tool list, stop and ask the user to update their MCP server configuration:
+>
+> ```json
+> {
+>   "mcpServers": {
+>     "chrome-devtools": {
+>       "command": "npx",
+>       "args": ["chrome-devtools-mcp@latest", "--categoryExtensions"]
+>     }
+>   }
+> }
+> ```
+>
+> After updating, the user must restart the MCP server (or their AI client) for the change to take effect.
+
 1. **Install**: Use `install_extension` with the path to the unpacked extension.
 2. **Identify**: Get the extension ID from the response or by calling `list_extensions`.
 3. **Trigger Action**: Use `trigger_extension_action` to open the popup or side panel if applicable.