docs: Add Rustdoc to gzip and user meta plugins, mark PLAN.md tasks done

Co-authored-by: aider (openai/andrew/openrouter/sonoma-sky-alpha) <aider@aider.chat>

src/meta_plugin/user.rs

@@ -1,6 +1,10 @@
 use crate::meta_plugin::{MetaPlugin, MetaPluginType};
 
 #[derive(Debug, Clone, Default)]
+/// Meta plugin for capturing current user and group information.
+///
+/// This plugin collects user ID, group ID, username, and group name for the process
+/// running the keep application, providing context about the creator of items.
 pub struct UserMetaPlugin {
     base: crate::meta_plugin::BaseMetaPlugin,
 }