docs(instructions): add explicit instruction not to modify CHANGELOG.md

[Copilot]

CHANGELOG.md is auto-generated during releases via release.sh using git commit history. Coding agents need explicit instructions not to modify it in pull requests.

Changes

Added instructions in three locations within .github/copilot-instructions.md:

• Root Directory Files: Updated CHANGELOG.md description to indicate it's auto-generated and should not be modified
• Before Committing checklist: Added item Added new benchmarks #7 explicitly prohibiting CHANGELOG.md modifications
• Common Pitfalls: Added item Added new benchmarks #7 explaining the automated release process and why manual edits are inappropriate

The instructions use emphatic language ("DO NOT MODIFY") to ensure clarity and prevent accidental updates during regular development work.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add instructions not to update CHANGELOG.md</issue_title>
<issue_description>CHANGELOG.md will be updated during the release process, and should not be modified in pull requests. Instructions for coding agents should contain this.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add instructions not to update CHANGELOG.md #734

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

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