core/cli: keep a prompt notification on screen

A prompt notification went out with the library defaults: the notification
server's own expiry. A desktop server drops such a notification after a few
seconds, so the announcement of a blocked turn had usually slid into the
notification tray by the time the human looked back.

It now goes out without expiry (`timeout=0`, the freedesktop value for "until
dismissed"). Urgency is left alone: measured on KDE Plasma, `critical` keeps the
notification up just the same, but paints it as urgent and asks the platform's
Focus mode to let it through — decoration a prompt does not need. Dismissing is
already the prompt's own job once it is answered, so nothing lingers.

typings/desktop_notifier/__init__.pyi

@@ -22,6 +22,11 @@ class Capability(Enum):
     THREAD = ...
     TIMEOUT = ...
 
+class Urgency(Enum):
+    Critical = ...
+    Normal = ...
+    Low = ...
+
 class Button:
     def __init__(
         self,
@@ -50,7 +55,7 @@ class DesktopNotifier:
         self,
         title: str,
         message: str,
-        urgency: Any = ...,
+        urgency: Urgency = ...,
         icon: Any = ...,
         buttons: Sequence[Button] = ...,
         reply_field: ReplyField | None = None,