add deploy/create/install apps, fix templates and docs

- Add apps: create (scaffold host from template), deploy (multi-host
  deployment with -n filter), install (NixOS installation from live ISO)
- Register all apps in flake.nix (create, deploy, install, rebuild)
- Add deploy.json config (cryodev-main, SSH port 2299)
- Fix generic-server template: was using Pi hardware/boot config,
  now correct x86_64 with systemd-boot, UEFI, ROOT/BOOT/SWAP labels
- Fix template networking.nix: use HOSTNAME placeholder instead of
  hardcoded cryodev-pi (both templates)
- Fix headplane upstream pnpm-deps hash mismatch via overlay
- Fix all docs: replace root@ with user@, --ssh-option with
  NIX_SSHOPTS, add deploy app references, update first-install guide
  to use create app and document service deactivation steps

apps/create/default.nix

@@ -0,0 +1,20 @@
+{
+  writeShellApplication,
+  git,
+  gnused,
+  ...
+}:
+
+let
+  name = "create";
+  text = builtins.readFile ./${name}.sh;
+in
+writeShellApplication {
+  inherit name text;
+  meta.mainProgram = name;
+
+  runtimeInputs = [
+    git
+    gnused
+  ];
+}