doc: Add AI agent guidance for the project

[qinxuye]

Summary

• Add a root AGENTS.md with project-specific guidance for AI coding agents
• Add CLAUDE.md as a symlink to AGENTS.md for Claude-compatible tooling
• Allow AGENTS.md and CLAUDE.md through the repository markdown ignore rule

Tests

• git diff --check
• pre-commit hooks during commit (skipped; no matching tracked hook files)

[gemini-code-assist]

Code Review

This pull request introduces AGENTS.md, a comprehensive guide for AI coding agents working on the Xinference project, and updates .gitignore to ensure it and CLAUDE.md are tracked. Feedback includes improving the document title for clarity, using npx for Prettier commands to ensure environment portability, and correcting CLAUDE.md to be a proper filesystem symbolic link rather than a plain text file.