Function rewrite_local_url_for_container 

pub fn rewrite_local_url_for_container(local_url: &str) -> String

Rewrite a host-loopback AGENTMUX_LOCAL_URL (e.g. http://127.0.0.1:PORT) into something a container’s network namespace can actually reach.

docker exec over the Docker socket never inherits the host process’s env the way a host subprocess does, so a container agent gets no sidecar URL at all unless one is explicitly injected — and injecting the loopback value verbatim would be reachable from the srv process but not from inside the container. host.docker.internal is resolvable from Docker Desktop (macOS/Windows) automatically, and from native Linux via the extra_hosts: host.docker.internal:host-gateway entry create_and_start sets — see #2939 workstream 1.

Only rewrites 127.0.0.1/localhost; any other host (already a real hostname/IP, or already host.docker.internal) passes through unchanged, so this is safe to call unconditionally.