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/build-liveusb.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+# Build Live USB ISO from flake configuration
+# Creates an uncompressed ISO suitable for Ventoy and other USB boot tools
+# Usage: nix run .#build-liveusb
+# Or: ./scripts/build-liveusb.sh
+
+set -euo pipefail
+
+REPO_ROOT="${REPO_ROOT:-$(git rev-parse --show-toplevel 2>/dev/null || pwd)}"
+
+echo "Building Live USB ISO..."
+nix build "${REPO_ROOT}#nixosConfigurations.live-usb.config.system.build.isoImage" --show-trace
+
+if ls "${REPO_ROOT}/result/iso/"*.iso 1> /dev/null 2>&1; then
+    iso_file=$(ls "${REPO_ROOT}/result/iso/"*.iso)
+    echo "Build complete!"
+    echo "ISO location: $iso_file"
+    echo "Ready for Ventoy or dd to USB"
+else
+    echo "Build failed - no ISO file found"
+    exit 1
+fi