nix-cache01: decommission and remove all references

Removed:
- hosts/nix-cache01/ directory
- services/nix-cache/build-flakes.{nix,sh} (replaced by NATS builder)
- Vault secret and AppRole for nix-cache01
- Old signing key variable from terraform
- Old trusted public key from system/nix.nix

Updated:
- flake.nix: removed nixosConfiguration
- README.md: nix-cache01 -> nix-cache02
- Monitoring rules: removed build-flakes alerts, updated harmonia to nix-cache02
- Simplified proxy.nix (no longer needs hostname conditional)

nix-cache02 is now the sole binary cache host.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

hosts/nix-cache01/configuration.nix

@@ -1,74 +0,0 @@
-{
-  pkgs,
-  ...
-}:
-
-{
-  imports = [
-    ./hardware-configuration.nix
-
-    ../../system
-    ../../common/vm
-  ];
-
-  homelab.host.role = "build-host";
-
-  fileSystems."/nix" = {
-    device = "/dev/disk/by-label/nixcache";
-    fsType = "xfs";
-  };
-  nixpkgs.config.allowUnfree = true;
-  # Use the systemd-boot EFI boot loader.
-  boot.loader.grub = {
-    enable = true;
-    device = "/dev/sda";
-    configurationLimit = 3;
-  };
-
-  networking.hostName = "nix-cache01";
-  networking.domain = "home.2rjus.net";
-  networking.useNetworkd = true;
-  networking.useDHCP = false;
-  services.resolved.enable = true;
-  networking.nameservers = [
-    "10.69.13.5"
-    "10.69.13.6"
-  ];
-
-  systemd.network.enable = true;
-  systemd.network.networks."ens18" = {
-    matchConfig.Name = "ens18";
-    address = [
-      "10.69.13.15/24"
-    ];
-    routes = [
-      { Gateway = "10.69.13.1"; }
-    ];
-    linkConfig.RequiredForOnline = "routable";
-  };
-  time.timeZone = "Europe/Oslo";
-
-  nix.settings.experimental-features = [
-    "nix-command"
-    "flakes"
-  ];
-  vault.enable = true;
-  homelab.deploy.enable = true;
-
-  nix.settings.tarball-ttl = 0;
-  environment.systemPackages = with pkgs; [
-    vim
-    wget
-    git
-  ];
-
-  services.qemuGuest.enable = true;
-
-  # Open ports in the firewall.
-  # networking.firewall.allowedTCPPorts = [ ... ];
-  # networking.firewall.allowedUDPPorts = [ ... ];
-  # Or disable the firewall altogether.
-  networking.firewall.enable = false;
-
-  system.stateVersion = "24.05"; # Did you read the comment?
-}

hosts/nix-cache01/default.nix

@@ -1,7 +0,0 @@
-{ ... }:
-{
-  imports = [
-    ./configuration.nix
-    ../../services/nix-cache
-  ];
-}

hosts/nix-cache01/hardware-configuration.nix

@@ -1,42 +0,0 @@
-{
-  config,
-  lib,
-  pkgs,
-  modulesPath,
-  ...
-}:
-
-{
-  imports = [
-    (modulesPath + "/profiles/qemu-guest.nix")
-  ];
-  boot.initrd.availableKernelModules = [
-    "ata_piix"
-    "uhci_hcd"
-    "virtio_pci"
-    "virtio_scsi"
-    "sd_mod"
-    "sr_mod"
-  ];
-  boot.initrd.kernelModules = [ "dm-snapshot" ];
-  boot.kernelModules = [
-    "ptp_kvm"
-  ];
-  boot.extraModulePackages = [ ];
-
-  fileSystems."/" = {
-    device = "/dev/disk/by-label/root";
-    fsType = "xfs";
-  };
-
-  swapDevices = [ { device = "/dev/disk/by-label/swap"; } ];
-
-  # Enables DHCP on each ethernet and wireless interface. In case of scripted networking
-  # (the default) this is the recommended approach. When using systemd-networkd it's
-  # still possible to use this option, but it's recommended to use it in conjunction
-  # with explicit per-interface declarations with `networking.interfaces.<interface>.useDHCP`.
-  networking.useDHCP = lib.mkDefault true;
-  # networking.interfaces.ens18.useDHCP = lib.mkDefault true;
-
-  nixpkgs.hostPlatform = lib.mkDefault "x86_64-linux";
-}