docs(agents): add AGENTS.md instructions for AI agents

[hajiboy95]

Summary
This PR adds AGENTS.md, which contains specific instructions, code guidelines, testing protocols, and formatting rules designed for AI agents working in this repository.

Why this is needed
As more developers use AI coding assistants and agents to build features and fix bugs in rift, it is important that they have clear, machine-readable repository guidelines. This ensures that agents automatically format code correctly, run testing commands, and adhere to structural conventions, maintaining high code quality and consistency.

Dependencies

• Complements the Git hooks PR: This documentation references and relies on the pre-commit/pre-push hooks currently being introduced in the active PR for branch add-githooks ci(githooks): add pre-commit and pre-push hooks #384.

[hajiboy95]

@acsandmann are you against such a file in your repo? If you don't want it I can close this PR