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-pi/sd-image.nix

@@ -0,0 +1,30 @@
+# SD Card image configuration for Raspberry Pi
+{
+  config,
+  modulesPath,
+  lib,
+  ...
+}:
+
+{
+  imports = [
+    (modulesPath + "/installer/sd-card/sd-image-aarch64.nix")
+  ];
+
+  sdImage = {
+    # Compress with zstd for smaller download
+    compressImage = true;
+
+    # Auto-expand root partition on first boot
+    expandOnBoot = true;
+  };
+
+  # Image filename based on hostname
+  image.fileName = "${config.networking.hostName}-sd-image.img";
+
+  # Disable ZFS to avoid build issues on SD image
+  boot.supportedFilesystems = lib.mkForce [
+    "vfat"
+    "ext4"
+  ];
+}