chore(prisma): enforce frontend-owned schema rollout policy

docs/prisma-governance.md

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+# Prisma governance
+
+## Single source of truth
+
+- Canonical Prisma schema: `frontend/prisma/schema.prisma`.
+- Service copies:
+  - `omni_chat/prisma/schema.prisma`
+  - `omni_outbound/prisma/schema.prisma`
+
+## Update flow
+
+1. Edit only `frontend/prisma/schema.prisma`.
+2. Run `./scripts/prisma-sync.sh`.
+3. Run `./scripts/prisma-check.sh`.
+4. Commit changed schema copies.
+
+## Rollout policy
+
+- Schema rollout (`prisma db push` / migrations) is allowed only in `frontend`.
+- `omni_chat` and `omni_outbound` must use generated Prisma client only.