docs: add available_tools field to extension config documentation (#7725)

documentation/docs/guides/config-files.md

@@ -122,6 +122,7 @@ extensions:
     name: "extension_name"    # Internal name
     timeout: 300              # Operation timeout in seconds
     type: "builtin"/"stdio"   # Extension type
+    available_tools: []       # Filter to specific tools (empty = all)
     
     # Additional settings for stdio extensions:
     cmd: "command"            # Command to execute
@@ -131,6 +132,10 @@ extensions:
     envs: {}                  # Environment values
 ```
 
+### Tool Filtering
+
+Use the `available_tools` field to limit which tools are loaded from an extension. List the tool names you want — only those will be available to goose. Leave it empty (the default) to load all tools. This can help reduce token overhead in sessions where you only need a subset of an extension's capabilities.
+
 ## Search Path Configuration
 
 Extensions may need to execute external commands or tools. By default, goose uses your system's PATH environment variable. You can add additional search directories in your config file: