docs: note macOS/Xcode-only build for cloud agents

[J2TeamNNL]

Summary

Attempted to set up the development environment for a Cloud Agent (Linux VM). TablePro is a native macOS/iOS app built with Xcode + XcodeGen, using AppKit, SwiftUI, and CloudKit. It cannot be built, tested, or run on Linux, and Xcode/AppKit cannot be installed on Linux.

This PR adds an AGENTS.md with a ## Cursor Cloud specific instructions section so future cloud agents immediately know the environment constraint and where the real build/test/lint commands live (README.md, CLAUDE.md).

What was verified

• VM is Ubuntu 24.04.4 LTS (x86_64).
• No Apple toolchain present or installable: xcodebuild, swift, swiftc, xcrun, xcodegen, swiftlint, swiftformat all missing.
• Packages/TableProCore declares only .macOS(.v14) / .iOS(.v17) and its core targets import AppKit/CloudKit, so swift build/swift test on Linux is not viable.
• Libs/ / Libs/ios/ are prebuilt macOS/iOS binaries, unusable without Xcode.

Not done (and why)

• No SetupVmEnvironment update script: there is no valid Linux dependency-install/refresh step for a macOS-only Xcode project.
• No app build/run/test artifacts: the application requires macOS + Xcode, which is unavailable on this VM.