Add repository setup notes

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Summary

Adds lightweight setup guidance for GitHub repository settings that cannot be fully captured in committed files.

Changes

• Adds docs/REPOSITORY_SETUP.md
• Covers default branch protection, required status checks, merge method guidance, auto-merge guidance, and release workflow expectations
• Adds practical setup order for maintainers creating a new repository from the template
• Links the setup notes from the README documentation map and repository settings section

Notes

This keeps the guidance generic and lightweight while making the final GitHub-side setup steps easier to find.

Closes #16