Function default_plugin_data_dir 

pub fn default_plugin_data_dir(plugin_name: &str) -> PathBuf

Client-managed persistent data directory for one installed plugin instance.

Uses VtCodePaths::data_dir()/plugin-data/<name> so state survives plugin updates (§9.1). Fallbacks (in order): platform data dir (dirs::data_dir()/vtcode/plugin-data/<name>), then the process temp dir. Callers must create it before spawn and preserve it across updates; it may be deleted on uninstall. Plugin names are charset-constrained (manifest.rs rejects /, \, ..), so join cannot escape.