bootstrap.sh: replace BASH_SOURCE with $0 to fix pipe execution

BASH_SOURCE[0] is unbound when bash reads from a pipe; set -u turns
this into a fatal error that no amount of :- or set +u reliably fixes
across bash versions. $0 is always set: 'bash' when piped (dirname
gives '.' where no setup.sh exists), and the correct path when run
directly.

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

bootstrap.sh

@@ -48,9 +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.
-set +u
-SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]:-}")" 2>/dev/null && pwd || echo "")"
-set -u
+SCRIPT_DIR="$(cd "$(dirname "$0")" 2>/dev/null && pwd || echo "")"
 if [ -f "${SCRIPT_DIR}/setup.sh" ]; then
     echo "  Running from local copy at $SCRIPT_DIR"
     echo "  (No git or internet needed)"