Simplify installer into shared common.nix + per-platform targets

Replaces the single monolithic installer.nix (preserved as
installer_old.nix for reference) with a shared common.nix carrying the
install-script/user/SSH baseline, consumed by per-platform targets
(installer.nix for netboot/ISO, new proxmox-lxc.nix for the Proxmox
LXC-based flow). flake.nix drops the nixos-generators input in favor
of building images directly from each nixosConfiguration's
system.build.isoImage/tarball.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

flake.lock

@@ -127,21 +127,6 @@
         "type": "github"
       }
     },
-    "nixlib": {
-      "locked": {
-        "lastModified": 1736643958,
-        "narHash": "sha256-tmpqTSWVRJVhpvfSN9KXBvKEXplrwKnSZNAoNPf/S/s=",
-        "owner": "nix-community",
-        "repo": "nixpkgs.lib",
-        "rev": "1418bc28a52126761c02dd3d89b2d8ca0f521181",
-        "type": "github"
-      },
-      "original": {
-        "owner": "nix-community",
-        "repo": "nixpkgs.lib",
-        "type": "github"
-      }
-    },
     "nixos-conf-editor": {
       "inputs": {
         "flake-compat": "flake-compat",
@@ -162,27 +147,6 @@
         "type": "github"
       }
     },
-    "nixos-generators": {
-      "inputs": {
-        "nixlib": "nixlib",
-        "nixpkgs": [
-          "nixpkgs"
-        ]
-      },
-      "locked": {
-        "lastModified": 1769813415,
-        "narHash": "sha256-nnVmNNKBi1YiBNPhKclNYDORoHkuKipoz7EtVnXO50A=",
-        "owner": "nix-community",
-        "repo": "nixos-generators",
-        "rev": "8946737ff703382fda7623b9fab071d037e897d5",
-        "type": "github"
-      },
-      "original": {
-        "owner": "nix-community",
-        "repo": "nixos-generators",
-        "type": "github"
-      }
-    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1771008912,
@@ -201,11 +165,11 @@
     },
     "nixpkgs_2": {
       "locked": {
-        "lastModified": 1784011430,
-        "narHash": "sha256-lDebytrYdd47IBLwvNOD+6AGeoqZ78CIKlp70hzW280=",
+        "lastModified": 1784280462,
+        "narHash": "sha256-DtoqIqM7VkR6NxAkcLpMwmi02USwWb3JdmNGLyhthc0=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "8eeec934ae0dbeca3d7868c059568a65c08b2fc3",
+        "rev": "293d6abedf0478e681a4dfcfcb35b30fc796a32f",
         "type": "github"
       },
       "original": {
@@ -221,7 +185,6 @@
         "flake-utils": "flake-utils",
         "home-manager": "home-manager",
         "nixos-conf-editor": "nixos-conf-editor",
-        "nixos-generators": "nixos-generators",
         "nixpkgs": "nixpkgs_2"
       }
     },