Add Ctrl+C quit handling to REPL

- Add _quit_requested flag to TerminalDisplay
- Add request_quit() method to TerminalDisplay
- Handle 'ctrl_c' key in REPL input loops in both pipeline_runner.py and main.py
- When Ctrl+C is pressed, request_quit() is called which sets the flag
- The main loop checks is_quit_requested() and raises KeyboardInterrupt

engine/app/pipeline_runner.py

@@ -986,7 +986,13 @@ def run_pipeline_mode(preset_name: str = "demo", graph_config: str | None = None
                 keys = display.get_input_keys(timeout=0.0)
 
                 for key in keys:
-                    if key == "return":
+                    if key == "ctrl_c":
+                        # Request quit when Ctrl+C is pressed
+                        if hasattr(display, "request_quit"):
+                            display.request_quit()
+                        else:
+                            raise KeyboardInterrupt()
+                    elif key == "return":
                         # Get command string before processing
                         cmd_str = repl_effect.state.current_command
                         if cmd_str: