Implement dynamic prompt symbol in PromptInput that changes based on the
first character of user input: '>' for normal mode, '/' for command mode,
and '$' for shell mode. Wire Textual event hooks for live updates, add CSS
modifier classes for mode-specific styling, and update consume_text() to
strip mode-symbol prefixes from submitted text. Add BDD scenario suite.

@@ -35,2 +35,3 @@
-* HAL 9000 has contributed the error-suppression removal fix (PR #9247 / issue #9060): removed both `try...except Exception:` blocks in `register_registry_agents()` that silently suppressed errors from `actor_registry.list_actors()` and the route bridge refresh, enabling exceptions to propagate per CONTRIBUTING.md fail-fast policy. Added three Behave scenarios verifying RuntimeError, AttributeError, and TypeError propagation.
+* HAL 9000 has contributed the error-suppression removal fix (PR #9247 / issue #9060): removed both `try...except Exception:` blocks in `register_registry_agents()` that silently suppressed errors from `actor_registry.list_actors()` and the route bridge refresh, enabling exceptions to propagate per CONTRIBUTING.md fail-fast policy. Added three Behave scenarios verifying RuntimeError, AttributeError, and TypeError propagation.
+* HAL 9000 has contributed the TUI prompt mode-dependent symbol fix (PR #6722 / issue #6431): implemented dynamic prompt symbol switching (`>` normal, `/` command, `$` shell) in `PromptInput`, wired Textual event hooks for live updates on input changes, added CSS modifier classes for mode-specific styling (accented border colors), and updated `consume_text()` to strip mode-symbol prefixes from submitted text. Added BDD test suite in `features/tui_prompt_mode_symbols.feature`.

@@ -0,0 +26,4 @@
+    """Reload the prompt module to reflect our changes."""
+    import importlib
+
+    mod = importlib.import_module(_PROMPT_MOD)

@@ -0,0 +1,142 @@
+Feature: PromptInput mode-dependent symbol (issue #6431)

@@ -41,0 +44,4 @@
+    | *(empty / any other text)*  | Normal    | '>'    |
+    | '/' (starts with slash)     | Command   | '/'    |
+    | '!' or '$' (starts with !)  | Shell     | '$'    |
+

@@ -41,0 +65,4 @@
+class PromptInput(_InputBase):
+    """TextArea widget wrapper with mode-dependent prompt symbol.
+
+    The prompt symbol changes dynamically based on the first non-whitespace

@@ -41,0 +129,4 @@
+    if hasattr(_InputBase, "on_input_changed"):  # type: ignore[arg-type]
+
+        def _prompt_on_input_changed(
+            self: PromptInput, *args: Any, **kwargs: Any

@@ -41,0 +152,4 @@
+# -----------------------------------------------------------------------
+
+if isinstance(_InputBase, type) and _InputBase is not object:
+