docs: add AI agent guide

[pollychen-lab]

Summary

• Add a root AGENTS.md for AI coding agents working in this Python boilerplate.
• Document the project layout, Makefile workflows, Ruff/pytest expectations, PR flow, and security guardrails.
• Point agents to existing canonical files instead of duplicating all setup details.

Closes #71

Testing

• git diff --check
• uvx poetry run ruff check .
• make local/install
• make local/lint
• make local/tests

Note: this environment does not have Poetry installed globally, so I used a temporary poetry shim that delegates to uvx poetry for the Makefile validation. Generated local artifacts (.env, poetry.lock, coverage.xml, htmlcov/) were removed and are not part of this PR.