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

hosts/server/host.nix

@@ -1,24 +0,0 @@
-{ vars, ... }:
-
-{
-  networking = {
-    hostName = vars.nfsServerHost;
-    hostId = "6689f93e";
-    useDHCP = false;
-    interfaces = {
-      ${vars.vmLanInterface}.ipv4.addresses = [{ address = vars.serverIp; prefixLength = vars.lanPrefixLength; }];
-      ${vars.vmStorageInterface}.ipv4.addresses = [{ address = vars.serverStorageIp; prefixLength = vars.haClientPrefixLength; }];
-    };
-    defaultGateway = { address = vars.lanGateway; interface = vars.vmLanInterface; };
-    nameservers = [ vars.domainControllerIp ];
-  };
-
-  services.beszel.agent.environment = {
-    EXTRA_FILESYSTEMS = "${vars.storageRoot}/${vars.nfsShares.dockerVolumes.subpath}";
-    LOG_LEVEL = "debug";
-  };
-
-  # Preserved from the pre-refactor `server` target — stateVersion must never
-  # be bumped on an already-installed machine.
-  system.stateVersion = "25.05";
-}