docs: convert Xcode prompts to Markdown

[cuyua9]

Summary

• rename the six existing Xcode prompt files from .txt to .md
• preserve the file contents exactly while improving GitHub Markdown rendering

Context

This is a small slice of #356, which asks to convert text prompt files to Markdown. I kept the scope limited to the self-contained Xcode/ folder so the change is easy to review.

Verification

• Compared each renamed .md file with its original .txt content from origin/main
• git diff --check

[coderabbitai]

Important

Review skipped

Review was skipped as selected files did not have any reviewable changes.

💤 Files selected but had no reviewable changes (6)

• Xcode/DocumentAction.md
• Xcode/ExplainAction.md
• Xcode/MessageAction.md
• Xcode/PlaygroundAction.md
• Xcode/PreviewAction.md
• Xcode/System.md

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 474625fb-9ebf-4d87-be76-97e355bd8391

📥 Commits

Reviewing files that changed from the base of the PR and between db9f7d8 and 61e3225.

📒 Files selected for processing (6)

• Xcode/DocumentAction.md
• Xcode/ExplainAction.md
• Xcode/MessageAction.md
• Xcode/PlaygroundAction.md
• Xcode/PreviewAction.md
• Xcode/System.md

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