Add REPL usage documentation and fix raw mode handling

- Fix raw mode enabling to not duplicate with UI border mode
- Add REPL_USAGE.md with comprehensive guide
- Add examples/repl_demo_terminal.py example script

engine/app/main.py

@@ -478,7 +478,8 @@ def run_pipeline_mode_direct():
             break
 
     # Enable raw mode for REPL if present and not already enabled
-    if repl_effect and hasattr(display, "set_raw_mode"):
+    # Also enable for UI border mode (already handled above)
+    if repl_effect and ui_panel is None and hasattr(display, "set_raw_mode"):
         display.set_raw_mode(True)
 
     # Run pipeline loop