feat: add metricflow_time_spine.yml to register time spine with MetricFlow

[dataders]

Summary

models/marts/metricflow_time_spine.sql exists but has no companion .yml to register it with MetricFlow. Without the .yml, the time spine is not recognized as a MetricFlow time spine model, so any metrics that depend on it won't resolve correctly.

This PR adds the missing .yml:

models:
  - name: metricflow_time_spine
    time_spine:
      standard_granularity_column: date_day
    columns:
      - name: date_day
        granularity: day

This is already present in the ClickHouse/jaffle-shop-clickhouse fork and is adapter-agnostic — it belongs upstream.