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.

secrets/home.client.admin.keyring

@@ -1,6 +0,0 @@
-[client.admin]
-	key = AQBojDlmfnc8MBAAkr+PXbSewmq4OooESo2X1A==
-	caps mds = "allow *"
-	caps mgr = "allow *"
-	caps mon = "allow *"
-	caps osd = "allow *"

secrets/home.mon.keyring

@@ -1,13 +0,0 @@
-[mon.]
-	key = AQAUijlm7emnJBAAKsHT1+2EzYRQxKsL4KwwkQ==
-	caps mon = "allow *"
-[client.admin]
-	key = AQBojDlmfnc8MBAAkr+PXbSewmq4OooESo2X1A==
-	caps mds = "allow *"
-	caps mgr = "allow *"
-	caps mon = "allow *"
-	caps osd = "allow *"
-[client.bootstrap-osd]
-	key = AQDvJDpm4BDlIhAAXISJWnrOtNDk0FqhSX0/YQ==
-	caps mgr = "allow r"
-	caps mon = "profile bootstrap-osd"

secrets/home.osd-bootstrap.keyring

@@ -1,4 +0,0 @@
-[client.bootstrap-osd]
-	key = AQDvJDpm4BDlIhAAXISJWnrOtNDk0FqhSX0/YQ==
-	caps mgr = "allow r"
-	caps mon = "profile bootstrap-osd"