fix: detect dollar prefix as shell mode trigger in InputModeRouter

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

features/steps/tui_input_modes_steps.py

@@ -143,3 +143,13 @@ def step_run_fallback_tui_app(context: Context) -> None:
 def step_fallback_tui_fails(context: Context, message: str) -> None:
     assert context.tui_fallback_error is not None
     assert message in str(context.tui_fallback_error)
+
+
+@when('I detect mode for "{text}"')
+def step_detect_mode(context: Context, text: str) -> None:
+    context.detected_mode = InputModeRouter.detect_mode(text)
+
+
+@then('the detected mode should be "{mode}"')
+def step_detected_mode_equals(context: Context, mode: str) -> None:
+    assert context.detected_mode.value == mode

features/tui_input_modes.feature

@@ -37,3 +37,23 @@ Feature: TUI input modes
     Then TUI textual availability should be boolean
     When I run fallback TUI app
     Then fallback TUI app should fail with "Textual dependency missing."
+
+  Scenario: Dollar prefix activates shell mode
+    When I route TUI input "$echo hello"
+    Then the TUI mode should be "shell"
+    And the TUI shell stdout should contain "hello"
+
+  Scenario: Dollar prefix detect_mode returns shell
+    When I detect mode for "$echo hello"
+    Then the detected mode should be "shell"
+
+
+  Scenario: Dollar prefix with leading whitespace activates shell mode
+    When I detect mode for "  $echo hello"
+    Then the detected mode should be "shell"
+
+
+  Scenario: Dollar prefix blocks dangerous command by default
+    When I route TUI input "$rm -rf /"
+    Then the TUI mode should be "shell"
+    And the TUI shell stderr should contain "blocked dangerous shell command"

src/cleveragents/tui/input/modes.py

@@ -51,7 +51,7 @@ class InputModeRouter:
         stripped = text.lstrip()
         if stripped.startswith("/"):
             return InputMode.COMMAND
-        if stripped.startswith("!"):
+        if stripped.startswith(("!", "$")):
             return InputMode.SHELL
         return InputMode.NORMAL