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>

docs/beszel.md

@@ -82,15 +82,6 @@ services.beszel.agent.environment = {
 };
 ```
 
-The `server` host uses this to expose its ZFS data pool:
-
-```nix
-services.beszel.agent.environment = {
-  EXTRA_FILESYSTEMS = "${vars.storageRoot}/${vars.nfsShares.dockerVolumes.subpath}";
-  LOG_LEVEL = "debug";
-};
-```
-
 ---
 
 ## Optional: monitoring Docker containers