docs: Add AGENTS.md

[jamsea]

• Add AGENTS.md following https://agents.md/ standard for coding agents
• Include comprehensive development environment setup instructions
• Add build/test commands, code style guidelines, and security considerations
• Provide project structure guidance and common patterns for contributors

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Copilot]

Pull Request Overview

This PR adds comprehensive documentation for coding agents following the https://agents.md/ standard. The documentation provides detailed setup instructions, development workflows, and contribution guidelines for the Pipecat AI framework.

• Adds AGENTS.md with development environment setup, build commands, and testing instructions
• Includes code style guidelines, project structure guidance, and security considerations
• Provides common patterns, performance guidelines, and troubleshooting information

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