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/)

apps/rebuild/default.nix

@@ -0,0 +1,30 @@
+{
+  writeShellApplication,
+  coreutils,
+  gnugrep,
+  gnused,
+  home-manager,
+  hostname,
+  nix,
+  nixos-rebuild,
+  ...
+}:
+
+let
+  name = "rebuild";
+  text = builtins.readFile ./${name}.sh;
+in
+writeShellApplication {
+  inherit name text;
+  meta.mainProgram = name;
+
+  runtimeInputs = [
+    coreutils
+    gnugrep
+    gnused
+    home-manager
+    hostname
+    nix
+    nixos-rebuild
+  ];
+}