xint-data-dir-symlink-required-for-upgrade-persistence

xint writes runtime data (api-costs.json, reliability-metrics.json) to a release-internal data/ directory by default, meaning data is wiped on every version upgrade. Fix: symlink releases/<version>/data → /opt/infra/xint/data/ (outside releases) immediately after install. Without this, cost history and reliability metrics reset on every upgrade.