serialise the agent-sockets and gateway publishes and give each its own tmp file

Closes #6094

hive-c0re/src/agent_set.rs

@@ -9,9 +9,12 @@ use std::path::Path;
 use anyhow::{Context, Result, bail};
 use tokio::sync::Mutex;
 
-/// Serialises [`refresh`]: `topology.json` is read, changed and written back,
-/// so two concurrent refreshes would lose one of the changes.
-static REFRESH: Mutex<()> = Mutex::const_new(());
+/// Serialises [`refresh`] and each tick of the 10-second poll in
+/// `workers::agent_sockets`. `topology.json` is read, changed and written
+/// back, so two concurrent refreshes would lose one of the changes; the poll
+/// holds it across list-and-write so it cannot publish an older agent list
+/// over one `refresh` just wrote.
+pub(crate) static REFRESH: Mutex<()> = Mutex::const_new(());
 
 /// Bring the host-side files in line with `agents`, the live agent set.
 ///