fix: prevent ScreenStackError when stopping agent from modal

[0xallam]

Summary

• Fix ScreenStackError crash when clicking the stop button in StopAgentScreen modal
• Move pop_screen() into on_button_pressed (shared by both stop and cancel paths) and remove the redundant pop in action_confirm_stop_agent, preventing a double-pop on a single-screen stack

[greptile-apps]

Greptile Summary

This PR fixes a ScreenStackError crash in StopAgentScreen by consolidating all modal dismissal into on_button_pressed — pop_screen() is now called unconditionally at the top of on_button_pressed and removed from action_confirm_stop_agent.

• on_button_pressed now calls self.app.pop_screen() before branching on which button was pressed, so both "Yes" (stop) and "No" (cancel) dismiss the modal via the same code path.
• action_confirm_stop_agent no longer calls self.pop_screen(), eliminating the error-prone pop inside the action.
• The escape key handler (line 251) still calls self.app.pop_screen() directly and is unaffected by this change.
• action_confirm_stop_agent is only called from on_button_pressed in this file, so removing pop_screen() from it has no other unintended side effects.

Confidence Score: 5/5

• This PR is safe to merge — the change is minimal, well-scoped, and correctly addresses the double-pop bug.
• The fix is straightforward: it moves a single pop_screen() call from action_confirm_stop_agent to the shared on_button_pressed handler. Both code paths (stop and cancel) correctly dismiss the modal exactly once. action_confirm_stop_agent has no other callers in the codebase, so removing pop_screen() from it has no unintended side effects. The escape key path remains independent and correct.
• No files require special attention.

Important Files Changed

Filename	Overview
strix/interface/tui.py	Consolidates pop_screen() into on_button_pressed (removing it from action_confirm_stop_agent) to fix a ScreenStackError caused by attempting a screen pop in an already-dismissed context. Both the "Yes" and "No" button paths are handled correctly.

_{Last reviewed commit: "fix: prevent ScreenS..."}