feat(node): SetManagement handler — merge FM managed-state into node config #169

Summary

Implement the SetManagement NodeService RPC (api #50) on the node: read the current persisted MachineConfig, merge the supplied Management block (nil clears/unlinks), and persist — a read-modify-write on the node so a LINK enrollment approval can mark the node managed WITHOUT ApplyConfig's whole-config replace. Admin-authorized. Unblocks manager Phase 3 LINK executor (manager #21). Shane 2026-07-16.