feat(emacs): Add prebuilt Doom option using nix-doom-emacs-unstraightened

Implement pre-built Doom Emacs packages for the live USB image, eliminating
the need to run `doom sync` after first boot.

Changes:
- Add nix-doom-emacs-unstraightened flake input
- Add homeModule to all three module sets (nixos, unstable, darwin)
- Add `prebuiltDoom` option to emacs role (default: false)
- Enable prebuiltDoom for live-usb configuration
- Pin custom packages in packages.el for deterministic builds:
  - claude-code-ide, gptel-tool-library, beads

When prebuiltDoom=true, all Doom packages are compiled at nix build time
using emacs-overlay. The doom configuration is stored in the nix store
(read-only), and no `doom sync` is required at runtime.

This is ideal for:
- Live USB images
- Immutable/reproducible systems
- Offline deployments

Closes: nixos-configs-1wd

flake.lock

@@ -21,6 +21,45 @@
         "type": "github"
       }
     },
+    "doomemacs": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1767773143,
+        "narHash": "sha256-QL/t9v2kFNxBDyNJb/s411o3mxujan+QX5IZglTdpTk=",
+        "owner": "doomemacs",
+        "repo": "doomemacs",
+        "rev": "3e15fb36d7f94f0a218bda977be4d3f5da983a71",
+        "type": "github"
+      },
+      "original": {
+        "owner": "doomemacs",
+        "repo": "doomemacs",
+        "type": "github"
+      }
+    },
+    "emacs-overlay": {
+      "inputs": {
+        "nixpkgs": [
+          "nix-doom-emacs-unstraightened"
+        ],
+        "nixpkgs-stable": [
+          "nix-doom-emacs-unstraightened"
+        ]
+      },
+      "locked": {
+        "lastModified": 1768011937,
+        "narHash": "sha256-SnU2XTo34vwVaijs+4VwcXTNwMWO4nwzzs08N39UagA=",
+        "owner": "nix-community",
+        "repo": "emacs-overlay",
+        "rev": "79abf71d9897cf3b5189f7175cda1b1102abc65c",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-community",
+        "repo": "emacs-overlay",
+        "type": "github"
+      }
+    },
     "flake-compat": {
       "flake": false,
       "locked": {
@@ -159,6 +198,27 @@
         "type": "github"
       }
     },
+    "nix-doom-emacs-unstraightened": {
+      "inputs": {
+        "doomemacs": "doomemacs",
+        "emacs-overlay": "emacs-overlay",
+        "nixpkgs": [],
+        "systems": "systems_2"
+      },
+      "locked": {
+        "lastModified": 1768034604,
+        "narHash": "sha256-62pIZMvGHhYJmMiiBsxHqZt/dFyENPcFHlJq5NJF3Sw=",
+        "owner": "marienz",
+        "repo": "nix-doom-emacs-unstraightened",
+        "rev": "9b3b8044fe4ccdcbb2d6f733d7dbe4d5feea18bc",
+        "type": "github"
+      },
+      "original": {
+        "owner": "marienz",
+        "repo": "nix-doom-emacs-unstraightened",
+        "type": "github"
+      }
+    },
     "nix-github-actions": {
       "inputs": {
         "nixpkgs": [
@@ -303,6 +363,7 @@
         "home-manager-unstable": "home-manager-unstable",
         "jovian": "jovian",
         "nix-darwin": "nix-darwin",
+        "nix-doom-emacs-unstraightened": "nix-doom-emacs-unstraightened",
         "nixos-wsl": "nixos-wsl",
         "nixpkgs": "nixpkgs_2",
         "nixpkgs-unstable": "nixpkgs-unstable",
@@ -324,6 +385,21 @@
         "repo": "default",
         "type": "github"
       }
+    },
+    "systems_2": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
     }
   },
   "root": "root",