Create or update an environment policy override

API key required. Sets the decision mode for one or more verbs in the {env} override (dev, staging, or prod). The override is sparse: only the verbs you send are stored; every other verb keeps inheriting the tenant default. Same body shapes and validation as PUT /v1/policy. Every successful update bumps that override’s version by one — the tenant default’s version is untouched. The response’s policyProvenance names the winning layer per verb.