Api Semantic Metrics

Metrics for the Semantic Layer page: the agent-authored semantic_models/metrics YAML for the active environment, parsed into flat records.

Metric definitions live in git (customer logic). Create/edit/delete apply DIRECTLY to BOTH environments (no PR), so editing is gated on GitHub being configured rather than on the env: editable (+ edit_reason) drives the catalog’s edit affordances regardless of which env is being viewed.