feat: add --ids-only flag to --list mode for scripting

Outputs one ID per line with no header. Errors if used with any mode
other than --list. Works with both local and client (remote) list.

src/args.rs

@@ -237,6 +237,10 @@ pub struct OptionsArgs {
     #[arg(help("Display file sizes with units"))]
     pub human_readable: bool,
 
+    #[arg(long)]
+    #[arg(help("Only output item IDs (for scripting)"))]
+    pub ids_only: bool,
+
     #[arg(short, long, action = clap::ArgAction::Count, conflicts_with("quiet"))]
     #[arg(help("Increase message verbosity, can be given more than once"))]
     pub verbose: u8,