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>

README.md

@@ -9,8 +9,8 @@ Targets are named `<platform>-<buildtype>`, generated from two orthogonal
 pieces composed in `flake.nix`:
 
 - **Platforms** (what it runs on): `linode`, `proxmox`, `lxc`, `baremetal`
-- **Build types** (what it's for): `minimal`, `nix-cache`, `server`, `docker`,
-  `gui`, `pxe-boot`, `tailscale-router`, `tor-relay`, `ha-server`
+- **Build types** (what it's for): `minimal`, `nix-cache`, `docker`, `gui`,
+  `pxe-boot`, `tailscale-router`, `tor-relay`, `ha-server`
 
 Not every combination exists — `pxe-boot` has no `linode` variant, since
 PXE/DHCP/TFTP need LAN L2 adjacency that a Linode VPS doesn't have,
@@ -24,7 +24,6 @@ hardware). The full list:
 | `proxmox-minimal` | Minimal NixOS host profile on Proxmox — previously the flat `nix-minimal` target |
 | `lxc-minimal` | Minimal NixOS host profile in a Proxmox LXC container |
 | `linode-nix-cache` / `proxmox-nix-cache` / `lxc-nix-cache` | Local Nix binary cache and remote builder — previously the flat `nix-cache` target |
-| `linode-server` / `proxmox-server` / `lxc-server` | Storage, NFS, backup, and monitoring exporter host — previously the flat `server` target |
 | `linode-docker` / `proxmox-docker` / `lxc-docker` | Docker host for the main container stack — previously the flat `docker` target |
 | `linode-gui` / `proxmox-gui` / `lxc-gui` | Cinnamon desktop workstation — previously the flat `nixos` target |
 | `baremetal-gui` | Same Cinnamon desktop workstation, on the real gui-host hardware — ZFS RAID0 root, systemd-boot |