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/)

flake.nix

@@ -10,7 +10,7 @@
     nixos-mailserver.url = "gitlab:simple-nixos-mailserver/nixos-mailserver";
     nixos-mailserver.inputs.nixpkgs.follows = "nixpkgs";
 
-    headplane.url = "github:yrd/headplane-nix";
+    headplane.url = "github:tale/headplane";
 
     comin.url = "github:nlewo/comin";
     comin.inputs.nixpkgs.follows = "nixpkgs";
@@ -41,7 +41,9 @@
 
       forAllSystems = nixpkgs.lib.genAttrs supportedSystems;
 
-      lib = nixpkgs.lib;
+      # Extend nixpkgs.lib with our custom utils
+      lib = nixpkgs.lib.extend (final: prev: self.lib or { });
+
       constants = import ./constants.nix;
 
       mkNixosConfiguration =
@@ -59,6 +61,26 @@
         };
     in
     {
+      # Custom library functions
+      lib = {
+        utils = import ./lib/utils.nix { lib = nixpkgs.lib; };
+      };
+
+      # Apps
+      apps = forAllSystems (
+        system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+          mkApp = name: {
+            type = "app";
+            program = pkgs.lib.getExe (pkgs.callPackage ./apps/${name} { });
+          };
+        in
+        {
+          rebuild = mkApp "rebuild";
+        }
+      );
+
       packages = forAllSystems (system: import ./pkgs nixpkgs.legacyPackages.${system});
 
       overlays = import ./overlays { inherit inputs; };