fix(prompt): suppress stale "(N behind)" version badge on local dev
The .commit-ref on a dev checkout can be a stale leftover from a
prior deploy, making the version API report a large behindBy count.
Use the `debug` flag (true on localhost and codespaces) to zero out
behindBy and remap "unknown" → "dev" — no user is deploying from a
dev tree, so the badge should just say "dev".
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>