Add grid agent milestone acceptance gate (#135)

crates/metacrate-grid-agent/README.md

@@ -68,3 +68,6 @@ The complete quick start, platform paths, systemd/Windows service operation,
 secret rotation, backup/upgrade/rollback, failure playbooks, resource defaults,
 and unsupported-operation list are in
 [`../../docs/grid-agent-operations.md`](../../docs/grid-agent-operations.md).
+Milestone acceptance, resource budgets, evidence, and opt-in live validation
+are defined in
+[`../../docs/grid-agent-acceptance.md`](../../docs/grid-agent-acceptance.md).