docs: add AGENTS.md

[harshitsinghbhandari]

Summary

• Adds a repo-root AGENTS.md tailored to the Go rewrite.
• Documents repo layout, build/test commands, first-read docs, coding conventions, hard architectural rules, and PR hygiene.
• Uses /tmp/ao-original/AGENTS.md only for shape/tone; content is specific to this repository.

Validation

• git diff --check -- AGENTS.md

Notes

• Separate PR from the project CLI commands work so the docs update does not block that feature.