Inherit parent thread settings when spawning subagents (#53328)

When a subagent was spawned while the parent had runtime-modified
settings (fast mode, thinking, etc.), the subagent would start with
defaults from `AgentSettings` instead of inheriting the parent's current
values.

This adds `inherit_parent_settings` as a single place in
`Thread::new_subagent` that copies all runtime-mutable settings from the
parent:

- `speed`
- `thinking_enabled`
- `thinking_effort`
- `summarization_model`
- `profile_id`

These are the same properties that `set_*` methods already propagate to
`running_subagents` — this fix ensures they're also inherited at
creation time.

cc @benbrandt

Release Notes:

- N/A

crates/agent/src/thread.rs

@@ -999,6 +999,7 @@ impl Thread {
             parent_thread_id: parent_thread.read(cx).id().clone(),
             depth: parent_thread.read(cx).depth() + 1,
         });
+        thread.inherit_parent_settings(parent_thread, cx);
         thread
     }
 
@@ -1085,6 +1086,19 @@ impl Thread {
         }
     }
 
+    /// Copies runtime-mutable settings from the parent thread so that
+    /// subagents start with the same configuration the user selected.
+    /// Every property that `set_*` propagates to `running_subagents`
+    /// should be inherited here as well.
+    fn inherit_parent_settings(&mut self, parent_thread: &Entity<Thread>, cx: &mut Context<Self>) {
+        let parent = parent_thread.read(cx);
+        self.speed = parent.speed;
+        self.thinking_enabled = parent.thinking_enabled;
+        self.thinking_effort = parent.thinking_effort.clone();
+        self.summarization_model = parent.summarization_model.clone();
+        self.profile_id = parent.profile_id.clone();
+    }
+
     pub fn id(&self) -> &acp::SessionId {
         &self.id
     }
@@ -4267,6 +4281,30 @@ mod tests {
         });
     }
 
+    #[gpui::test]
+    async fn test_subagent_inherits_settings_at_creation(cx: &mut TestAppContext) {
+        let (parent, _event_stream) = setup_thread_for_test(cx).await;
+
+        cx.update(|cx| {
+            parent.update(cx, |thread, cx| {
+                thread.set_speed(Speed::Fast, cx);
+                thread.set_thinking_enabled(true, cx);
+                thread.set_thinking_effort(Some("high".to_string()), cx);
+                thread.set_profile(AgentProfileId("custom-profile".into()), cx);
+            });
+        });
+
+        let subagents = setup_parent_with_subagents(cx, &parent, 1);
+
+        cx.update(|cx| {
+            let sub = subagents[0].read(cx);
+            assert_eq!(sub.speed(), Some(Speed::Fast));
+            assert!(sub.thinking_enabled());
+            assert_eq!(sub.thinking_effort().map(|s| s.as_str()), Some("high"));
+            assert_eq!(sub.profile(), &AgentProfileId("custom-profile".into()));
+        });
+    }
+
     #[gpui::test]
     async fn test_set_speed_propagates_to_subagents(cx: &mut TestAppContext) {
         let (parent, _event_stream) = setup_thread_for_test(cx).await;