Adds .NET SDK port

[Kusp00110110]

• document repository purpose and build/test workflow in AGENTS instructions
• add .NET solution with base, system, and affiliate clients
• include unit test covering request signing

[Kusp00110110]

@moh3a

I work primarily in the .NET ecosystem, especially within the NopCommerce open-source community where developers can create plugins for the store. I envisioned building an AliExpress plugin and figured there must already be an SDK available in some form. That’s when I came across your repo.

This PR comes from my fork of your master branch. Using Codex, I reconstructed the foundations of your TypeScript SDK into a usable .NET port. At this stage it’s a simplified version with the basic features, but I plan to flesh it out over time. I’ll continue maintaining the .NET port and will submit additional PRs as things progress. Eventually, I’d like to add a NopCommerce plugin and submit it to their repo, giving credit back to your work.

Thanks again for the effort you put into the TypeScript SDK—hoping this addition makes it easier for .NET developers to leverage its capabilities.