[Open Source] Enable manual triggering of system prompt builds (#859)

Author: jordanhunt22

.github/workflows/release-system-prompts.yml renamed to .github/workflows/release_system_prompts.yml

@@ -6,6 +6,7 @@ on:
       - main
     paths:
       - 'chef-agent/prompts/**'
+  workflow_dispatch: # Allow manual triggering
 
 permissions:
   contents: write