Pull request overview

This PR adds an AGENTS.md file to provide guidance for AI coding agents working on the MyST-Parser repository, along with supporting tox environments for code quality tooling.

Changes:

• Added comprehensive AGENTS.md documentation covering project structure, development workflows, architecture, and best practices
• Added tox environments for ruff linting/formatting and mypy type checking
• Added dependency groups for mypy and ruff in pyproject.toml

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
AGENTS.md	New comprehensive guide for AI agents with project overview, development commands, architecture documentation, and coding guidelines
tox.ini	Added ruff-check, ruff-fmt, and mypy test environments for code quality tooling
pyproject.toml	Added dependency groups for mypy and ruff with pinned versions

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