Stateless time sensor. On each scheduled tick it publishes a {kind: 'tick'}
marker to the agent's sense_data channel. The chat loop dispatches on the
marker, runs _check_and_fire_concerns against the current wall clock, and
autonomously executes any concern instructions whose cadence has elapsed.
This is the trigger source for chat-mode Phase C autonomy. Wall clock is the single source of truth — the sensor does not advance any internal clock; it just removes the user-turn dependency from cadence checks.