refactor: full repo sweep — variables, docs, and comment cleanup

- variables.nix: switch to rec {}, extract giteaDomain/giteaRepoPath,
  extraAdminSshKeys, haLanNfsFqdn, tailscaleResolverIp, ports.dhcp,
  ports.dns; ipaServer now derives from homeDomain ref; section headers
- modules: use new vars throughout (pxe-boot, ts-dns-forwarder,
  cluster-config, configuration.nix, mount-pxe-images) — eval unchanged
- docs: delete ephemeral planning docs (AUDIT_REPORT, ha-network-audit,
  network-cutover); add docs/ha.md; drop migration reference table from
  ip-addressing.md; remove stale server example from beszel.md
- CLAUDE.md/README.md/AGENTS.md: fix build types (tailscale-router,
  ha-server, drop server); document scripts/ha/, scripts/ipa/, and
  all previously undocumented top-level and lib scripts

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/ha/cluster-config.nix

@@ -42,12 +42,8 @@ let
   '';
 in
 {
-  # Root SSH access — same key set as nixos user so all admin keys can reach root.
-  users.users.root.openssh.authorizedKeys.keys = [
-    vars.adminSshKey
-    "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAICMJhrfFayLBG+gWtO6oAvgambw5nWWgztiTFEaaaVRH debian@surface"
-    "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIGygkCljN6uKpdJbHTOQtn8ZnH+wKXDLAwrDFbLrE/65 nixos@nixos"
-  ];
+  # Root SSH access — same key set as the nixos user so all admin keys can reach root.
+  users.users.root.openssh.authorizedKeys.keys = [ vars.adminSshKey ] ++ vars.extraAdminSshKeys;
 
   # Passwordless sudo for wheel — operator SSHes as nixos and uses sudo for
   # cluster management commands (drbdadm, crm*, pcs, etc.)