Human availability grants

Availability grants are human-controlled policies. A grant selects external calendars and applies their opaque busy intervals to selected agents, all agents that existed when the grant was created, or all current and future agents.

Grant mutations require a console session. Agent availability remains provider-neutral and returns availability_state, source health, and fail-closed empty slots when a required human source is stale, partial, or unavailable. allow_stale=true is for explicitly non-committing exploration.