Keep parent tab highlighted when focus is on a sub-agent

The top tab bar compared against focusedID, so stepping into a
sub-agent dropped the parent tab's highlight even though the user
was still inside that thread. activeAgentID already walks the
parent chain to the top-level root for the sidebar's agent tree
— reuse it for the tab strip too.

internal/app/tabbar.go

@@ -24,7 +24,11 @@ func (st *uiState) drawTabBar() {
 	}
 	st.mu.Lock()
 	palOpen := st.palette != nil
-	focus := st.focusedID
+	// Highlight the top-level agent tab even when focus has stepped
+	// into a sub-agent (or a Processes pane entry). activeAgentID walks
+	// the parent chain to the root, so the user always sees which tab
+	// their current thread belongs to.
+	focus := st.activeAgentID
 	st.mu.Unlock()
 	if palOpen {
 		return