agent: Auto-capture telemetry feature flag (#28271)

Release Notes:

- N/A

crates/feature_flags/src/feature_flags.rs

@@ -95,6 +95,15 @@ impl FeatureFlag for Debugger {
     const NAME: &'static str = "debugger";
 }
 
+pub struct ThreadAutoCapture {}
+impl FeatureFlag for ThreadAutoCapture {
+    const NAME: &'static str = "thread-auto-capture";
+
+    fn enabled_for_staff() -> bool {
+        false
+    }
+}
+
 pub trait FeatureFlagViewExt<V: 'static> {
     fn observe_flag<T: FeatureFlag, F>(&mut self, window: &Window, callback: F) -> Subscription
     where