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

scripts/bootstrap.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+# bootstrap.sh
+# Usage: nix run .#bootstrap -- <hostname>
+# Or: sudo ./scripts/bootstrap.sh <hostname>
+set -euo pipefail
+
+NEW_HOSTNAME="${1:?missing hostname}"
+FLAKE_URI="git+https://git.johnogle.info/johno/nixos-configs.git#${NEW_HOSTNAME}"
+
+export NIX_CONFIG="experimental-features = nix-command flakes"
+nixos-rebuild switch --flake "$FLAKE_URI"