bootstrap.sh: use set +u guard around BASH_SOURCE to fix pipe execution

${BASH_SOURCE[0]:-} still triggers set -u when BASH_SOURCE is entirely
unset (not just empty) in pipe mode. Temporarily disable -u for that
single assignment, then restore it.

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

bootstrap.sh

@@ -48,7 +48,9 @@ echo ""
 
 # ── Option 3: already running from inside the repo ───────────────────────────
 # If setup.sh is sitting next to this script, we have everything we need.
+set +u
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]:-}")" 2>/dev/null && pwd || echo "")"
+set -u
 if [ -f "${SCRIPT_DIR}/setup.sh" ]; then
     echo "  Running from local copy at $SCRIPT_DIR"
     echo "  (No git or internet needed)"