Function write_shared_from_env 

pub fn write_shared_from_env(agent_id: &str, local_url: &str, block_id: &str)

Convenience wrapper over write_shared: resolves the shared dir and this process’s channel itself, no-op if the shared root can’t be resolved. Exists so call sites outside server/reactive.rs’s explicit register/unregister handlers (PTY shell auto-register, persistent stream-json controller auto-register — both call the per-channel write/remove directly, not through the HTTP handler) don’t each repeat the same resolve-dir-then-read-env dance.