docs: add AGENTS.md for coding-agent onboarding

[yuchen814]

Why are these changes needed?

Adds a root AGENTS.md so AI coding assistants (Claude Code, Copilot, etc.) have a single, concise entry point for working in this monorepo. It captures the non-obvious essentials that are otherwise spread across several docs:

• Package managers per toolchain (uv for python/, dotnet for dotnet/, yarn for the Studio frontend).
• File-/package-scoped test, lint, format, type-check and build commands for both the Python (poe) and .NET (dotnet) sides.
• Pointers to the existing setup/architecture/security/release docs instead of duplicating them (.github/copilot-instructions.md, python/README.md, dotnet/README.md, dotnet/PACKAGING.md, CONTRIBUTING.md, docs/design/, SECURITY.md).
• Repo conventions inferred from the docs: unified autogen-* versioning, Google-style + Sphinx RST docstrings, pytest with ReplayChatCompletionClient/skipif, legacy AutoGen.* vs new Microsoft.AutoGen.*, and PR-template/CLA expectations.

The file is intentionally short (~57 lines) and references existing documentation rather than restating it, so it stays cheap to maintain.

Related issue number

N/A

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

Note: this PR only adds a top-level Markdown file (AGENTS.md); it introduces no code, so no tests are applicable. Opened as a draft for maintainer review of the content and conventions.

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