refactor: resolve module conflict and reorganize meta plugin structure

Co-authored-by: aider (openai/andrew/openrouter/qwen/qwen3-coder) <aider@aider.chat>

src/meta_plugin/mod.rs

@@ -1,13 +1 @@
-pub mod cwd;
-pub mod user;
-pub mod shell;
-pub mod shell_pid;
-pub mod keep_pid;
-pub mod hostname;
-
-pub use cwd::CwdMetaPlugin;
-pub use user::UserMetaPlugin;
-pub use shell::ShellMetaPlugin;
-pub use shell_pid::ShellPidMetaPlugin;
-pub use keep_pid::KeepPidMetaPlugin;
-pub use hostname::HostnameMetaPlugin;
+pub mod system;