Add SD image pipeline, documentation overhaul, and fix module issues

- Add automatic SD image builds for Raspberry Pi via Forgejo Actions
- Enable binfmt emulation on cryodev-main for aarch64 cross-builds
- Add sd-image.nix module to cryodev-pi configuration
- Create comprehensive docs/ structure with installation guides
- Split installation docs into: first-install (server), reinstall, new-client (Pi)
- Add lib/utils.nix and apps/rebuild from synix
- Fix headplane module for new upstream API (tale/headplane)
- Fix various module issues (mailserver stateVersion, option conflicts)
- Add placeholder secrets.yaml files for both hosts
- Remove old INSTRUCTIONS.md (content moved to docs/)

hosts/cryodev-main/services/headscale.nix

@@ -1,6 +1,4 @@
 {
-  config,
-  pkgs,
   outputs,
   constants,
   ...
@@ -17,7 +15,9 @@
     port = constants.services.headscale.port;
     settings = {
       server_url = "https://${constants.services.headscale.fqdn}";
-      dns_config.base_domain = constants.domain;
+      # dns.base_domain must be different from the server domain
+      # Using "tail" for internal Tailscale DNS (e.g., host.tail)
+      dns.base_domain = "tail";
     };
   };