grpcctl: add auth login and policy commands

- Add auth/login and auth/logout to mciasgrpcctl, calling
  the existing AuthService.Login/Logout RPCs; password is
  always prompted interactively (term.ReadPassword), never
  accepted as a flag, raw bytes zeroed after use
- Add proto/mcias/v1/policy.proto with PolicyService
  (List, Create, Get, Update, Delete policy rules)
- Regenerate gen/mcias/v1/ stubs to include policy
- Implement internal/grpcserver/policyservice.go delegating
  to the same db layer as the REST policy handlers
- Register PolicyService in grpcserver.go
- Add policy list/create/get/update/delete to mciasgrpcctl
- Update mciasgrpcctl man page with new commands

Security: auth login uses the same interactive password
prompt pattern as mciasctl; password never appears in
process args, shell history, or logs; raw bytes zeroed
after string conversion (same as REST CLI and REST server).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/grpcserver/grpcserver.go

@@ -120,6 +120,7 @@ func (s *Server) buildServer(extra ...grpc.ServerOption) *grpc.Server {
 	mciasv1.RegisterTokenServiceServer(srv, &tokenServiceServer{s: s})
 	mciasv1.RegisterAccountServiceServer(srv, &accountServiceServer{s: s})
 	mciasv1.RegisterCredentialServiceServer(srv, &credentialServiceServer{s: s})
+	mciasv1.RegisterPolicyServiceServer(srv, &policyServiceServer{s: s})
 
 	return srv
 }