feat: Move bootstrap/build-liveusb scripts to flake apps

- Move bootstrap.sh to scripts/ and add as flake app
- Move build-liveusb.sh to scripts/ and add as flake app
- Update usage comments to show nix run commands
- Improve build-liveusb.sh with better error handling (set -euo pipefail)
- Remove emojis from output messages for cleaner log output

Scripts can now be run consistently via:
  nix run .#bootstrap -- <hostname>
  nix run .#build-liveusb

Implements bead: nixos-configs-bli

flake.nix

@@ -275,6 +275,16 @@
           export PATH="${pkgs.lib.makeBinPath commonDeps}:$PATH"
           ${builtins.readFile ./scripts/upgrade.sh}
         '';
+
+        bootstrap = pkgs.writeShellScriptBin "bootstrap" ''
+          export PATH="${pkgs.lib.makeBinPath commonDeps}:$PATH"
+          ${builtins.readFile ./scripts/bootstrap.sh}
+        '';
+
+        build-liveusb = pkgs.writeShellScriptBin "build-liveusb" ''
+          export PATH="${pkgs.lib.makeBinPath commonDeps}:$PATH"
+          ${builtins.readFile ./scripts/build-liveusb.sh}
+        '';
       in {
         update-doomemacs = {
           type = "app";
@@ -292,6 +302,14 @@
           type = "app";
           program = "${upgrade}/bin/upgrade";
         };
+        bootstrap = {
+          type = "app";
+          program = "${bootstrap}/bin/bootstrap";
+        };
+        build-liveusb = {
+          type = "app";
+          program = "${build-liveusb}/bin/build-liveusb";
+        };
       }
     );
   };