Refine Filesystem billing guidance and make telemetry TiDB-only

[Icemap]

Summary

• surface structured Filesystem free-tier quota errors with an actionable TiDB Cloud billing link
• make TiDB the only telemetry storage destination and remove PostHog configuration, forwarding, readiness, metrics, and tests
• expand the Metabase dashboard SQL with canonical ti command names, Lab scope filtering, and detailed error drill-down queries
• keep telemetry E2E compatible with the legacy test DSN variable while validating the real CLI-to-backend-to-TiDB path
• update the Drive9 and Filesystem reference submodules to the revisions used during this work

Testing

• make test
• make e2e
• make telemetry-e2e