fix: add missing YAML separator to changelog prompt; default model to gpt-5.4-mini

Author: esokullu

.github/prompts/minor-release-changelog.prompt.yml

@@ -18,3 +18,4 @@ messages:
 modelParameters:
   maxCompletionTokens: 1200
   temperature: 0.2
+---

.github/workflows/minor-release.yml

@@ -17,7 +17,7 @@ on:
         description: GitHub Models model for AI changelog generation.
         required: false
         type: string
-        default: openai/gpt-4o
+        default: openai/gpt-5.4-mini
 
 permissions:
   contents: write