cli/notify: notify a waiting prompt through the desktop or the terminal

A prompt that has waited `grace_seconds` is now announced, and where the
platform allows it the notification carries the answer: allow / deny / review
for a confirmation, a reply field for an `ask_user_*` question. Picking
"review" withdraws the notification and keeps waiting at the terminal, and a
notification is dismissed as soon as the prompt returns — answered or not.

Two channels. The desktop one is the optional `notify` extra, run on its own
event loop in a daemon thread because a notification's callbacks only fire
while the loop that owns it is running (the library's synchronous wrapper does
not provide that). The terminal one needs nothing: it writes OSC 777 / OSC 9
to `/dev/tty` — so a redirect cannot swallow it — picking the protocol from the
environment and wrapping it for tmux / screen, falling back to a plain bell.
kitty speaks OSC 99 only, which is not implemented, so it keeps the bell.

Nothing here can break a prompt: a missing extra, a desktop that refuses, a
failed write all fall through to the next channel and never raise. Titles are
flattened and stripped of control characters, and the reason line (which can
name a command or a URL) only travels with `include_reason`.

typings/desktop_notifier/__init__.pyi

@@ -0,0 +1,66 @@
+# Minimal stubs for the optional ``desktop-notifier`` extra.
+# Only the surface ``plyngent.cli.notify`` uses is declared; the real package is
+# installed with ``pip install plyngent[notify]`` and is absent by default.
+
+from collections.abc import Sequence
+from enum import Enum
+from typing import Any
+
+class Capability(Enum):
+    APP_NAME = ...
+    TITLE = ...
+    MESSAGE = ...
+    URGENCY = ...
+    ICON = ...
+    BUTTONS = ...
+    REPLY_FIELD = ...
+    ATTACHMENT = ...
+    ON_DISPATCHED = ...
+    ON_CLICKED = ...
+    ON_DISMISSED = ...
+    SOUND = ...
+    THREAD = ...
+    TIMEOUT = ...
+
+class Button:
+    def __init__(
+        self,
+        title: str,
+        on_pressed: Any = None,
+        identifier: str | None = None,
+    ) -> None: ...
+
+class ReplyField:
+    def __init__(
+        self,
+        title: str = ...,
+        button_title: str = ...,
+        on_replied: Any = None,
+    ) -> None: ...
+
+class DesktopNotifier:
+    def __init__(
+        self,
+        app_name: str = ...,
+        app_icon: Any = ...,
+        notification_limit: int | None = None,
+    ) -> None: ...
+    async def get_capabilities(self) -> frozenset[Capability]: ...
+    async def send(
+        self,
+        title: str,
+        message: str,
+        urgency: Any = ...,
+        icon: Any = ...,
+        buttons: Sequence[Button] = ...,
+        reply_field: ReplyField | None = None,
+        on_dispatched: Any = None,
+        on_clicked: Any = None,
+        on_dismissed: Any = None,
+        attachment: Any = ...,
+        sound: Any = ...,
+        thread: str | None = None,
+        timeout: int = -1,  # noqa: ASYNC109 - the library's own parameter name
+    ) -> str: ...
+    async def clear(self, identifier: str) -> None: ...
+    async def clear_all(self) -> None: ...