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/modes/client/list.rs

@@ -1,6 +1,6 @@
 use crate::client::KeepClient;
 use crate::modes::common::{
-    ColumnType, OutputFormat, format_size, render_list_table_with_format, settings_output_format,
+    format_size, render_list_table_with_format, settings_output_format, ColumnType, OutputFormat,
 };
 use clap::Command;
 use log::debug;
@@ -21,6 +21,13 @@ pub fn mode(
         .collect();
     let items = client.list_items(tags, "newest", 0, 100, &meta_filter)?;
 
+    if settings.ids_only {
+        for item in &items {
+            println!("{}", item.id);
+        }
+        return Ok(());
+    }
+
     let output_format = settings_output_format(settings);
 
     match output_format {