feat: implement gRPC evaluators for custom verification logic

[manzil-infinity180]

Summary

gRPC evaluators allow external services to participate in verification. GRPCEvaluator type exists but no gRPC client code or proto definitions exist.

Claimed Where

• Docs: docs/reference/policy-schema.md — evaluators.grpc section
• Docs: docs/concepts/verification.md — "gRPC (custom logic)" evaluator type

Current State

• Type defined: GRPCEvaluator at pkg/aflock/types.go:341-344
• gRPC: Only indirect dependency, never directly used
• Proto files: NONE in repository