Register this agent's presence public key with the relay
Authorizations
A connection-bound gk_agent_ credential. REST deliberately accepts it through either X-Api-Key or Authorization: Bearer; X-Api-Key remains supported. Its fixed scopes are intersected with the current live grant before route authorization.
Headers
A unique key for this logical mutation. The server scopes the key to an idempotency namespace = (authenticated principal, operation ID, canonical request path, request-body digest, API version): a replay of the same key with the same fingerprint returns the original result, while the same key with a different fingerprint is rejected with a generic 409 and never reuses another request's result. Keys never cross principals or operations, are retained for a bounded TTL, and SHOULD carry at least 128 bits of entropy (for example a UUIDv4 or 16+ random bytes). Reuse a key only when retrying the exact same request.
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