feat: add gstack-upload helper for Supabase Storage

lib/upload.ts provides uploadScreenshot() that uploads to Supabase
Storage and returns a public CDN URL. Falls back gracefully to local
path with stderr warning on any failure (no config, expired auth,
network error). Exit code 0 always — never breaks calling templates.

bin/gstack-upload is a thin bash wrapper for CLI use.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

bin/gstack-upload

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+# gstack-upload — upload a file to Supabase Storage, print public URL
+# Falls back to local path on failure (exit 0 always)
+set -euo pipefail
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+exec bun run "$SCRIPT_DIR/../lib/upload.ts" "$@"