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

home/home-live-usb.nix

@@ -14,7 +14,12 @@
     desktop.enable = true;
     tmux.enable = true;
     plasma-manager.enable = true;
-    emacs.enable = true;
+    emacs = {
+      enable = true;
+      # Use pre-built Doom Emacs - all packages built at nix build time
+      # This means no doom sync is needed after booting the live USB
+      prebuiltDoom = true;
+    };
     i3_sway.enable = true;
     starship.enable = true;
     # development.enable = false;  # Not needed for live USB