feat: decommission server.sweet.home

Removes all flake artifacts for the retired NFS/ZFS file server:
- flake.nix: drop linode-server, proxmox-server, lxc-server targets
- hosts/server/: delete host identity
- modules/build-types/server.nix: delete build type (ZFS pool + NFS exports)
- vars/per-machine/{linode,lxc,proxmox}-server/: delete clan vars
- secrets/server.keytab: delete IPA keytab
- .sops.yaml: remove server key anchors, strip from common.yaml recipients,
  delete server.yaml and server.keytab creation rules
- variables.nix: remove serverIp, serverStorageIp (nfsServerHost/storageRoot
  retained — pxe-boot/mount-pxe-images.nix still references them)
- docs: mark server decommissioned in ip-addressing.md

Docker NFS mounts already pointed at nfs.storage.home (previous commit).
VM destruction and DNS cleanup (server.sweet.home, server.storage.home
A/PTR records) to follow once the VM is confirmed stopped.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J8djTWdXVzXZc99iujU6T2

flake.nix

@@ -109,9 +109,6 @@
         proxmox-nix-cache = mkTarget { platform = "proxmox"; buildType = "nix-cache"; hostPath = ./hosts/nix-cache/host.nix; };
         lxc-nix-cache = mkTarget { platform = "lxc"; buildType = "nix-cache"; hostPath = ./hosts/nix-cache/host.nix; };
 
-        linode-server = mkTarget { platform = "linode"; buildType = "server"; hostPath = ./hosts/server/host.nix; };
-        proxmox-server = mkTarget { platform = "proxmox"; buildType = "server"; hostPath = ./hosts/server/host.nix; };
-        lxc-server = mkTarget { platform = "lxc"; buildType = "server"; hostPath = ./hosts/server/host.nix; };
 
         linode-docker = mkTarget { platform = "linode"; buildType = "docker"; hostPath = ./hosts/docker/host.nix; };
         proxmox-docker = mkTarget { platform = "proxmox"; buildType = "docker"; hostPath = ./hosts/docker/host.nix; };