Docs: a recipe for an isolated background service #508

Channel only selects a registration, and a released CLI registers the shared service.json. So isolating a background service means separate XDG roots (state and config must differ; see #475), an explicit port, and a guarded Stop. None of that is written down as one recipe.

Proposed: one runnable recipe in the connection-modes guide, beside the state/config warning.

Source: internal developer-experience evaluation of the 2.0.32 nightly, 2026-10-18 (finding D32-10).