docs(agents): add hierarchical AGENTS.md guides

[mym0404]

Type of change

• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Enhance (enhance performance, api, etc)
• Chore
• This change requires a documentation update
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

What does this change?

• Add a hierarchical AGENTS.md knowledge base at root and major domain directories (src, ios, android, example, docs, expo-config-plugin, script).
• Define per-directory ownership, conventions, anti-patterns, and validation commands to reduce cross-domain mistakes.
• Document Fabric/codegen parity expectations and platform-specific guidance for native implementation areas.

No linked issue.