Add reusable cleanup prompt

Author: webprofusion-chrisc

docs/prompts/cleanup.prompt.md

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+Cleanup code:
+- Removing unnecessary comments and ensuring the project file is concise. Retain XML doc comments.
+- Add comments for clarity where needed.
+- Ensure the code is formatted correctly and follows best practices. The following rules should also be applied:
+    - Leave a newline before and after if. Do not use single line if and instead use opening and closing braces.
+    - In general, leave a newline between code that is not directly related.