Add buf lint/breaking targets and fix proto naming violations

- Add buf.yaml with STANDARD lint rules and FILE-level breaking change detection
- Add proto-lint Makefile target (buf lint + buf breaking --against master)
- Add lint Makefile target (golangci-lint) and include it in all
- Fix proto target: use module= option so protoc writes to gen/ not proto/
- engine.proto: rename rpc Request→Execute and message types accordingly
- acme.proto: drop redundant ACME prefix from SetConfig/ListAccounts/ListOrders messages
- policy.proto: add CreatePolicyResponse/GetPolicyResponse wrappers instead of returning PolicyRule directly from multiple RPCs
- Update grpcserver and webserver/client.go to match renamed types

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

buf.yaml

@@ -0,0 +1,17 @@
+version: v2
+
+modules:
+  # Setting path to proto/ means buf sees files at metacrypt/v1/*.proto,
+  # which matches the package name metacrypt.v1 as required by PACKAGE_DIRECTORY_MATCH.
+  - path: proto
+
+lint:
+  use:
+    - STANDARD
+
+breaking:
+  use:
+    # FILE mode is the strictest: catches wire-breaking changes (removed/renumbered
+    # fields, changed field types) as well as source-breaking changes (removed RPCs,
+    # renamed messages). Appropriate for a service with external gRPC clients.
+    - FILE