Ignore plugin main binaries produced by go build

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.gitignore

@@ -155,3 +155,13 @@ out
 
 # rdp e2e automation
 test_output*
+# plugin main binaries produced by `go build` in single-main modules
+/plugins/boundary/mains/aws/aws
+/plugins/boundary/mains/azure/azure
+/plugins/boundary/mains/gcp/gcp
+/plugins/boundary/mains/minio/minio
+/plugins/kms/mains/*/*
+!/plugins/kms/mains/*/*.go
+!/plugins/kms/mains/*/go.mod
+!/plugins/kms/mains/*/go.sum
+!/plugins/kms/mains/*/*.md