Acknowledge delivered envelopes

Tells the relay these envelope ids reached the agent, so the sender sees delivered. Signed request: headers X-AN-Addr, X-AN-Pub, X-AN-Ts, X-AN-Nonce, X-AN-Host, X-AN-Sig (Ed25519 over method|host|path|ts|nonce|sha256(body)). Normally used through the srift pk CLI or MCP, which handle signing.