core/config: add the [notify] section

`[notify]` decides what the CLI does when a prompt is waiting on the human:
`grace_seconds` (default 5s, roughly Claude Code's permission notice) before
anything is shown, `min_interval_seconds` between two notifications,
`channels` (`auto` = desktop then terminal), `actions` for the allow/deny/review
buttons, `include_reason` for the one-line reason, and `protocol` to pin the
terminal escape protocol.

The reason stays out by default because it names commands, argv and URLs, and a
notification outlives the prompt and may be mirrored to a lock screen. Unknown
keys degrade to the defaults, the way `[mcp]` does, and `reload()` re-reads the
section.

Desktop notifications need the optional `notify` extra (`desktop-notifier`),
which is the only new dependency and stays off the default install.

pyproject.toml

@@ -22,6 +22,10 @@ requires-python = ">=3.14,<4"
 readme = "README.md"
 license = { text = "MIT" }
 
+[project.optional-dependencies]
+# Desktop notifications while a prompt waits on the human (see `[notify]`).
+notify = ["desktop-notifier>=6.2.0"]
+
 [project.scripts]
 plyngent = "plyngent.cli:main"