docs(desktop): document polkit agent configuration and package requirements

docs/desktop/umbriel.mdx

@@ -5,6 +5,8 @@ description: Umbriel scrolling compositor config — Noctalia-only session, niri
 
 [Umbriel](https://docs.noctalia.dev/umbriel/) is a wlroots scrolling compositor from the Noctalia project. The config ports the [niri](/desktop/niri) layout, rules, springs, and binds, and adds the per-app scratch workspaces from [Hyprland](/desktop/hyprland) as Umbriel scratchpads. The session starts only `noctalia`: its launcher, clipboard history, polkit agent, notifications, and OSD replace vicinae, cliphist, and polkit-gnome.
 
+Noctalia's polkit agent is off by default. `dot_config/noctalia/config.toml` (→ `~/.config/noctalia/config.toml`, merged under the GUI's unmanaged `~/.local/state/noctalia/settings.toml`) sets `[shell] polkit_agent = true`, so `pkexec` and other privileged actions prompt in a Noctalia panel. `polkitd` itself is D-Bus activated (`polkit.service` is static; nothing to enable). The setting also applies under niri, which still starts polkit-gnome: only one agent registers per session, whichever starts first.
+
 :::note
 Linux `desktop` and `laptop` roles only (`.chezmoiignore.tmpl`). `umbriel` and its portal backend `xdg-desktop-portal-umbriel` (screenshots and screen sharing for portal clients) come from the `[cachyos]` repo (`packages.arch.desktop.cachyos`); the AUR `umbriel` is a placeholder and `umbriel-git` conflicts with the repo build, so other Arch machines install them by hand.
 :::