The spec requires both ! and dollar sign to activate shell mode, but
detect_mode() only recognised ! as the shell mode trigger. This caused
dollar-prefixed inputs like dollar-sign-echo-hello to be routed to NORMAL
mode instead of SHELL mode.
- Add dollar sign to detect_mode() shell prefix check alongside !
- Add BDD tests for dollar prefix shell mode detection
- Add step definitions for detect_mode() direct testing
Closes#10412