Function write_shared 

pub fn write_shared(
    shared_dir: &Path,
    agent_id: &str,
    local_url: &str,
    block_id: &str,
    channel: &str,
)

Write (create or update) this channel’s entry for agent_id in the host-global shared registry. Writes only this channel’s own file (shared_channel_path) — never touches another channel’s file, so concurrent writers for different channels of the same agent name cannot race or clobber each other.