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/)

modules/nixos/nixvim/plugins/treesitter.nix

@@ -30,12 +30,12 @@ in
     };
 
     # Fix for: ERROR `cc` executable not found.
-    home.sessionVariables = mkIf plugin.enable {
+    environment.sessionVariables = mkIf plugin.enable {
       CC = mkDefault cc;
     };
 
     # Fix for: WARNING `tree-sitter` executable not found
-    home.packages = mkIf plugin.enable [
+    environment.systemPackages = mkIf plugin.enable [
       plugin.package
     ];
   };