Add central Nix builder and binary cache

modules/applications/nix-fleet/meta.nix

@@ -0,0 +1,3 @@
+{
+  description = "Fleet build and deploy command-line tools";
+}

modules/applications/nix-fleet/nixos.nix

@@ -0,0 +1,63 @@
+{
+  inputs,
+  pkgs,
+  primaryUser,
+  ...
+}:
+let
+  system = pkgs.stdenv.hostPlatform.system;
+  deployRs = inputs.deploy-rs.packages.${system}.default;
+
+  fleetBuild = pkgs.writeShellApplication {
+    name = "fleet-build";
+    runtimeInputs = [
+      pkgs.jq
+      pkgs.nix
+    ];
+    text = ''
+      flake_ref="''${FLAKE:-/home/${primaryUser}/dotfiles}"
+
+      if (( $# == 0 )); then
+        # Keep this pipeline inside command substitution so pipefail and
+        # writeShellApplication's errexit propagate evaluation failures.
+        host_lines="$(
+          nix eval --json "$flake_ref#nixosConfigurations" \
+            --apply 'configs: builtins.attrNames configs' |
+            jq -r '.[] | select(. != "installer")'
+        )"
+        if [[ -z "$host_lines" ]]; then
+          echo "No deployable NixOS hosts found in $flake_ref" >&2
+          exit 1
+        fi
+        mapfile -t hosts <<< "$host_lines"
+      else
+        hosts=("$@")
+      fi
+
+      for host in "''${hosts[@]}"; do
+        nix build \
+          --out-link "/var/lib/nix-fleet/roots/build/$host" \
+          "$flake_ref#nixosConfigurations.$host.config.system.build.toplevel"
+      done
+    '';
+  };
+
+  fleetDeploy = pkgs.writeShellApplication {
+    name = "fleet-deploy";
+    runtimeInputs = [ deployRs ];
+    text = ''
+      cd "''${FLAKE:-/home/${primaryUser}/dotfiles}" || exit 1
+
+      exec deploy \
+        --keep-result \
+        --result-path /var/lib/nix-fleet/roots/deploy \
+        "$@"
+    '';
+  };
+in
+{
+  environment.systemPackages = [
+    fleetBuild
+    fleetDeploy
+  ];
+}