fix: prevent ScreenStackError when stopping agent from modal

Move pop_screen() into on_button_pressed before dispatching the stop
action, and remove the redundant pop in action_confirm_stop_agent.
This avoids a double-pop that crashes when the screen stack has only
the default screen left.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

strix/interface/tui.py

@@ -252,10 +252,9 @@ def on_key(self, event: events.Key) -> None:
             event.prevent_default()
 
     def on_button_pressed(self, event: Button.Pressed) -> None:
+        self.app.pop_screen()
         if event.button.id == "stop_agent":
             self.app.action_confirm_stop_agent(self.agent_id)
-        else:
-            self.app.pop_screen()
 
 
 class VulnerabilityDetailScreen(ModalScreen):  # type: ignore[misc]
@@ -1912,8 +1911,6 @@ def _validate_agent_for_stopping(self) -> tuple[str, bool]:
         return agent_name, False
 
     def action_confirm_stop_agent(self, agent_id: str) -> None:
-        self.pop_screen()
-
         try:
             from strix.tools.agents_graph.agents_graph_actions import stop_agent