Publish agent spawns atomically

Status: accepted

Agent derives a unique canonical path from the free-form task name and reserves it with one agent-turn capacity slot atomically before publishing a child. Collisions with retained or preparing identities select a numbered sibling rather than failing; see ADR 0034. Exhausted capacity or initialization failure leaves no registry entry and releases both reservations, so the unpublished path remains reusable. The tool returns the path only after the agent record and initial turn are committed. Provider or tool failure after publication settles that turn as errored and releases capacity, while the durable agent identity and path remain available for follow-up.