Api Create Semantic Metric

Create a governed metric and apply it DIRECTLY to both environments (no PR). Body: {name, label?, description?, mart (ref('mart_x') or 'mart_x'), grain?, agg, expr, status?}. The definition (agg over expr) is validated by RUNNING it against the staging marts in the bound warehouse before anything is written; on success the metric (and its measure) are added to that mart's semantic YAML in both the staging and prod workspaces and committed directly to both git branches. Requires GitHub configured. Permission: create_proposals.