Document markdownkv output format in StackQL constructor

Added 'markdownkv' to the list of available output formats in the
StackQL class constructor documentation. This format was added in
commit 52d089e but was not documented in the constructor parameters.

Author: claude

pystackql/core/stackql.py

@@ -32,8 +32,8 @@ class StackQL:
     :param backend_file_storage_location: Specifies location for database file, only applicable when `backend_storage_mode` is 'file'
         (defaults to `'{cwd}/stackql.db'`, this option is ignored in `server_mode`)
     :type backend_file_storage_location: str, optional
-    :param output: Determines the format of the output, options are 'dict', 'pandas', and 'csv' 
-        (defaults to `'dict'`, `'csv'` is not supported in `server_mode`)
+    :param output: Determines the format of the output, options are 'dict', 'pandas', 'csv', and 'markdownkv'
+        (defaults to `'dict'`, `'csv'` is not supported in `server_mode`, 'markdownkv' is optimized for LLM understanding)
     :type output: str, optional
     :param sep: Seperator for values in CSV output 
         (defaults to `','`, `output='csv'` only)