RLM loop

In-tree verify/act loop: castellan run --rlm.

Sandbox seam (RlmSandboxHost)

Behind castellan-rlm feature sandbox — stub for future Modal/Daytona backends:

#![allow(unused)]
fn main() {
use castellan_rlm::{RlmSandboxHost, SandboxRequest, StubSandboxHost};
}

Not wired into run_until_satisfied yet.