[Enhancement] vela auth has no way to revoke privileges granted by grant-privileges

Is there an existing issue for this?

• I have searched the existing issues

Affected feature or component

vela auth CLI command group

What is the current limitation or pain point?

vela auth has grant-privileges and list-privileges, but no revoke-privileges. The underlying auth.RevokePrivileges function in pkg/auth/privileges.go is fully implemented and already covered by TestGrantAndRevokePrivileges in pkg/auth/privileges_test.go — it just has zero callers anywhere in the codebase, so it's never exposed to users.

Today the only way to revoke a privilege granted via vela auth grant-privileges is to manually edit the underlying RoleBinding/ClusterRoleBinding with kubectl, which defeats the purpose of the vela auth abstraction in the first place.

Describe the improvement you'd like

Add vela auth revoke-privileges, mirroring grant-privileges's flag surface (--user/--group/--serviceaccount/--kubeconfig/--for-cluster/--for-namespace/ --readonly) minus --create-namespace (not applicable to revoke), calling the existing auth.RevokePrivileges.

Alternatives considered

None — the implementation this needs already exists and is tested; this is pure CLI wiring.

Additional context

No response