bootstrap.sh: fix BASH_SOURCE unbound variable when piped through bash

set -euo pipefail causes ${BASH_SOURCE[0]} to abort with 'unbound variable'
when the script is fed via curl | bash. Use ${BASH_SOURCE[0]:-} so the
variable expands to an empty string in that context, letting SCRIPT_DIR
resolve safely and the pipe path continue to the git-clone branch.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LQJBvqzXeyuhhAcAA3Q5Wq

bootstrap.sh

@@ -48,7 +48,7 @@ echo ""
 
 # ── Option 3: already running from inside the repo ───────────────────────────
 # If setup.sh is sitting next to this script, we have everything we need.
-SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" 2>/dev/null && pwd || echo "")"
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]:-}")" 2>/dev/null && pwd || echo "")"
 if [ -f "${SCRIPT_DIR}/setup.sh" ]; then
     echo "  Running from local copy at $SCRIPT_DIR"
     echo "  (No git or internet needed)"