Move default_time_dimension from Metric to Model

[nicosuave]

Summary

Breaking change: default_time_dimension and default_grain are now model-level settings, not per-metric.

This aligns with MetricFlow's approach (defaults.agg_time_dimension) where all metrics in a semantic model share the same default time dimension.

Changes

• Removed default_time_dimension and default_grain from Metric
• Added default_time_dimension and default_grain to Model
• Generator auto-includes default time dimension when querying metrics
• Updated MetricFlow adapter to use defaults.agg_time_dimension
• Updated Sidemantic adapter for model-level defaults
• Updated Omni adapter to use model's default

Example

models:
  - name: orders
    default_time_dimension: order_date
    default_grain: month
    dimensions:
      - name: order_date
        type: time
    metrics:
      - name: revenue
        agg: sum
        sql: amount

# Auto-includes orders.order_date__month
layer.compile(metrics=["orders.revenue"])

# Override with different granularity
layer.compile(metrics=["orders.revenue"], dimensions=["orders.order_date__week"])