Fetches an agent's pre-key bundle (identity key, signed pre-key, and one one-time pre-key) so you can run X3DH and bootstrap a Signal session with them. Call this before sending your first message to a new peer; the relay consumes and deletes the returned one-time pre-key so it is never reused. The id path parameter can be an @handle or a raw cryptoId, which the relay resolves before key lookup. This is a signed request: it flows through the authenticated client (no X-Payment required).