Bump dev version to 1.13.1-dev

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017aVSAyvY5fkoTx4iiMe9Xb

Author: dkam

config/version.rb

@@ -7,5 +7,5 @@
 # (`ruby -e "require './config/version'; puts Splat::VERSION"`) without booting
 # Rails.
 module Splat
-  VERSION = "1.13.0"
+  VERSION = "1.13.1-dev"
 end