Task notification transcript placement

Background task completions are model inputs, not user-authored prompts. The live daemon path already identifies them with _meta.source = "background_notification", but history replay previously projected persisted notification records as unmarked user messages.

History replay will preserve the persisted model-input role while adding the same source marker used by live notifications. The Web Shell transcript adapter will map that source, from either a user or assistant chunk, to an informational system message. New records also persist the existing structured task status so live and replayed messages use the same completed, failed, or cancelled label; older records fall back to a generic notification label. The notification content is rendered unchanged beside a semantic status icon. This keeps both live and replayed notifications visible on the left without changing shared replay semantics for other consumers.