Add PR description length guidelines to agent instructions

[Copilot]

Adds guidelines requiring PR descriptions to be at most 3 sentences, with specific guidance for breaking changes.

Changes:

• Added "Pull Request Description Guidelines" section to AGENTS.md under Contributing
• Created .github/copilot-instructions.md with PR description best practices
• Both files specify 3-sentence maximum and require 1-2 sentences describing broken APIs for breaking changes

Example from new guidelines:

**Keep PR descriptions concise and focused:**
- Describe your changes in **at most 3 sentences**
- Focus on the what and why, not implementation details
- If the PR breaks public-facing APIs, use one or two sentences to describe what is broken and how users should adapt

Original prompt

Add instrucitons to both copilot-instructions and agents.md to ask Copilot to only describe at most 3 sentences in PR descriptions. If the PR is breaking public-facing APIs, use one or two sentences to describe what are broken.

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