Attach an existing gate as a sub-gate of an agent gate

Use when: caller has an existing gate they want to attach as a sub-gate of an agent gate (rather than creating a new one)

Attaches an already-existing gate (owned by the caller) as a sub-gate of the agent gate at :id. Rejects self-attachment, double-attachment, and attachments where the sub-gate is locked to a different agent gate.