Type Alias NotesHandler 

pub type NotesHandler = fn(vault: PathBuf, frontier: PathBuf, backend: Option<String>, max_files: Option<usize>, max_items_per_category: Option<usize>, dry_run: bool, json: bool) -> Pin<Box<dyn Future<Output = ()> + Send>>;

v0.23 Agent Inbox: pluggable handler for vela compile-notes. Same OnceLock pattern as the scout handler; the binary registers it at startup.