ci-runner: warn when the host has no nix store gc

docs/4-implementation/scheduler/ci.md

@@ -314,8 +314,10 @@ CI day can fill the disk until every job fails fast with `ENOSPC`.
 
 Store GC is a **host-level** concern, so it belongs in the host's own NixOS
 configuration, not in the hyperhive service modules — a single service should
-not reach out and change the host's global nix-daemon options. Add the
-following to your host config:
+not reach out and change the host's global nix-daemon options. The module
+only warns at `nixos-rebuild` time when the host sets neither
+`nix.gc.automatic` nor `nix.settings.min-free`. Add the following to your
+host config:
 
 ```nix
 {

docs/examples/semi-spread/swarm-heavyduty.nix

@@ -32,6 +32,18 @@ in
     ];
   };
 
+  # CI builds land in this host's store; the runner module warns without a
+  # collector. See docs/4-implementation/scheduler/ci.md, "Host store
+  # maintenance", for the daily timer and the disk-pressure settings.
+  nix.gc = {
+    automatic = true;
+    dates = "daily";
+    randomizedDelaySec = "45min";
+    options = "--delete-older-than 1d";
+  };
+  nix.settings.min-free = 20 * 1024 * 1024 * 1024;
+  nix.settings.max-free = 50 * 1024 * 1024 * 1024;
+
   # The cache is served through the gateway, over the VPN only.
   networking.firewall.interfaces.wg0.allowedTCPPorts = [ hyperhive.gateway.httpsPort ];
 }