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>

internal/webserver/client.go

@@ -159,7 +159,7 @@ func (c *VaultClient) Mount(ctx context.Context, token, name, engineType string,
 
 // EngineRequest sends a generic engine operation to the vault.
 func (c *VaultClient) EngineRequest(ctx context.Context, token, mount, operation string, data map[string]interface{}) (map[string]interface{}, error) {
-	req := &pb.EngineRequest{
+	req := &pb.ExecuteRequest{
 		Mount:     mount,
 		Operation: operation,
 	}
@@ -170,7 +170,7 @@ func (c *VaultClient) EngineRequest(ctx context.Context, token, mount, operation
 		}
 		req.Data = s
 	}
-	resp, err := c.engine.Request(withToken(ctx, token), req)
+	resp, err := c.engine.Execute(withToken(ctx, token), req)
 	if err != nil {
 		return nil, err
 	}