Add agent instructions

[Snuffy2]

Summary

Adds a standalone AGENTS.md for the upstream repository so coding agents have accurate, project-specific guidance without relying on local environment instructions.

What Changed

• Documented the current MouseTrap stack, repository layout, persistent-data defaults, and SQLite-backed event log.
• Added backend, frontend, documentation, testing, security, and pull-request handoff guidance.
• Listed local setup and validation commands aligned with the repository Python 3.13 and Node.js 22.20+ tooling.

Why

The upstream repository does not currently provide project-level instructions for coding agents. This gives contributors a durable reference for making scoped changes while preserving persistent user data, security boundaries, and the existing development workflow.

[sirjmann92]

Thanks @Snuffy2 , good idea.