fix: change callback_type to prepend_lines for doc command

When generating documentation, the default replacement behavior
replaces the code with the document. Changed to prepend_lines
so documentation is added above the code instead of replacing it.

Author: zzhirong

lua/codegpt/config.lua

@@ -58,6 +58,7 @@ local default_commands = {
 			cpp = "Use doxygen style comments for functions.",
 			java = "Use JavaDoc style comments for functions.",
 		},
+		callback_type = "prepend_lines",
 	},
 	opt = {
 		user_message_template = "I have the following {{language}} code: ```{{filetype}}\n{{text_selection}}```\nOptimize this code. {{language_instructions}} Only return the code snippet and nothing else.",