Status bar reads directly from Agent and MindState on each render tick.
Activity is now a field on Agent — set by agent code directly, read by
UI via try_lock. DmnAnnotation, ContextInfoUpdate, AgentUpdate were
already dead (no senders).
UiMessage down to 4 variants: TextDelta, Reasoning, Debug, Info.
Co-Authored-By: Proof of Concept <poc@bcachefs.org>