c0re: emit /var/lib/hyperhive/agent-ports.json on meta sync for gateway (#15)

hive-c0re/src/meta.rs

@@ -101,6 +101,20 @@ pub async fn sync_agents(
     let topology_changed = crate::topology::reconcile(&agent_names)
         .with_context(|| format!("reconcile {}", crate::topology::topology_path().display()))?;
 
+    // Refresh /var/lib/hyperhive/agent-ports.json so the hive-gateway
+    // nginx sees the new agent set (#15 / #740). The file is the
+    // single source of truth for which agents the gateway proxies to,
+    // since the gateway container lives in system config and can't be
+    // rebuilt from meta-flake events. Atomic write (tmp + rename) so
+    // a partial write never trips the gateway's read.
+    if let Err(e) = crate::agent_ports::write(&agent_names) {
+        // Best-effort: a failed write doesn't block the meta-flake
+        // regen + container ops that follow. The gateway falls back
+        // to whatever map is currently on disk (or an empty map on
+        // first boot, meaning no per-agent routing yet).
+        tracing::warn!(error = ?e, "agent_ports::write failed (non-fatal)");
+    }
+
     if initial {
         git(&dir, &["init", "--initial-branch=main"]).await?;
     }