chore: clean up builder2, Ceph module, normalize Darwin host symlinks

- Remove builder2 (retired host): dangling network.json entry and
  empty home/hosts/builder2 stub. Its old IP (10.42.1.17) is already
  correctly owned by pve4.
- Remove the abandoned Ceph module (modules/nixos/ceph.nix) and its
  unencrypted plaintext keyring files under secrets/. No host ever
  enabled services.ceph-benaco; the keyrings were dead, unencrypted
  credentials sitting in the repo.
- Normalize Darwin host identity: IVR and Lithic are the only two
  physical Darwin machines. All other darwin/hosts/* names are now
  symlinks to whichever of the two they represent, matching the DHCP
  name variations nix-darwin sees depending on network:
    gregory -> ivr
    gregory.hellings-mbp -> ivr
    MacBook-Prolocal -> ivr
    gregs-MacBook-Pro-16-inch-Nov-2024 -> lithic
    li -> lithic (pre-existing)
  This lets each machine's config be maintained once regardless of
  what hostname it currently advertises.

darwin/hosts/ivr/default.nix

@@ -0,0 +1,59 @@
+{ ... }:
+let
+  username = "gregory.hellings";
+in
+{
+  greg = {
+    nix.cache = false;
+  };
+
+  homebrew = {
+    enable = true;
+    brews = [
+      "bitwarden-cli"
+      "direnv"
+      {
+        name = "libvirt";
+        restart_service = true;
+      }
+      "nushell"
+      "poetry"
+      {
+        name = "postgresql@17";
+        restart_service = true;
+      }
+      "pytest"
+      "qemu"
+    ];
+    casks = [
+      "alt-tab"
+      "audacity"
+      "bitwarden"
+      "bruno"
+      "chromium"
+      "claude"
+      "dbeaver-community"
+      "ghostty"
+      "firefox"
+      "microsoft-teams"
+      "notunes"
+      "onlyoffice"
+      "pgadmin4"
+      "podman-desktop"
+      "tabby"
+      "vagrant"
+      "virtualbox"
+      "visual-studio-code"
+      "zed"
+      "zoho-workdrive"
+    ];
+    user = username;
+  };
+
+  system.primaryUser = username;
+
+  users.users."${username}" = {
+    name = username;
+    home = "/Users/${username}";
+  };
+}