Add AGENTS.md with guidance for AI coding agents

[lahma]

Adds an AGENTS.md describing what an AI agent has to know to be useful in this repository, and a CLAUDE.md that only imports it (@AGENTS.md) so Claude Code and other agents read the same file instead of each getting their own copy.

Contents:

• Commands — the NUKE targets, plus the faster plain-SDK loop and the -f net8.0 filter for running a single fixture without repeating everything on net462/net472. Notes that the version comes from the top of CHANGELOG.md and that warnings are errors.
• Architecture — how WithJs() wires into AngleSharp, one engine per window, that the exposed JS surface follows the services registered in the browsing context, the four proxy types, and the type-to-prototype canonicalization that makes instanceof work.
• Performance — that it is a primary concern here, the cache-everything-reflective and do-it-on-first-use patterns the recent work established, the .NET techniques that apply, and the netstandard2.0/net462 constraint that rules out Span<T> and friends without a package reference.
• Conventions — the parts of CONTRIBUTING.md that differ from ordinary modern C# (using inside the namespace, String over string, ConfigureAwait(false)).
• Tests — the EvalScriptAsync idiom, the network-dependent fixtures, and a warning that Constants.cs is ~750 KB of embedded jQuery/React.

Documentation only; no code changes.

🤖 Generated with Claude Code

[FlorianRappl]

LGTM