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>

proto/generate.go

@@ -6,5 +6,5 @@
 //
 // Prerequisites: protoc, protoc-gen-go, protoc-gen-go-grpc must be in PATH.
 //
-//go:generate protoc --proto_path=../proto --go_out=../gen --go_opt=paths=source_relative --go-grpc_out=../gen --go-grpc_opt=paths=source_relative mcias/v1/common.proto mcias/v1/admin.proto mcias/v1/auth.proto mcias/v1/token.proto mcias/v1/account.proto
+//go:generate protoc --proto_path=../proto --go_out=../gen --go_opt=paths=source_relative --go-grpc_out=../gen --go-grpc_opt=paths=source_relative mcias/v1/common.proto mcias/v1/admin.proto mcias/v1/auth.proto mcias/v1/token.proto mcias/v1/account.proto mcias/v1/policy.proto
 package proto