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.nix

@@ -47,6 +47,12 @@
       url = "github:steveyegge/beads";
       inputs.nixpkgs.follows = "nixpkgs-unstable";
     };
+
+    nix-doom-emacs-unstraightened = {
+      url = "github:marienz/nix-doom-emacs-unstraightened";
+      # Don't follow nixpkgs to avoid rebuild issues with emacs-overlay
+      inputs.nixpkgs.follows = "";
+    };
   };
 
   outputs = { self, nixpkgs, nixpkgs-unstable, nixos-wsl, ... } @ inputs: let
@@ -70,6 +76,7 @@
         home-manager.useUserPackages = true;
         home-manager.sharedModules = [
           inputs.plasma-manager.homeModules.plasma-manager
+          inputs.nix-doom-emacs-unstraightened.homeModule
         ];
         home-manager.extraSpecialArgs = {
           globalInputs = inputs;
@@ -98,6 +105,7 @@
         home-manager.useUserPackages = true;
         home-manager.sharedModules = [
           inputs.plasma-manager-unstable.homeModules.plasma-manager
+          inputs.nix-doom-emacs-unstraightened.homeModule
         ];
         home-manager.extraSpecialArgs = {
           globalInputs = inputs;
@@ -129,6 +137,9 @@
         ];
         home-manager.useGlobalPkgs = true;
         home-manager.useUserPackages = true;
+        home-manager.sharedModules = [
+          inputs.nix-doom-emacs-unstraightened.homeModule
+        ];
         home-manager.extraSpecialArgs = {
           globalInputs = inputs;
         };