Document iOS/Xcode dev environment setup (macOS-only)

[aj327]

Summary

CheckList is a native iOS app (Swift 3.0, UIKit + Storyboards + Core Data) built with Xcode. It has no third-party dependency manager (no CocoaPods/SPM/Carthage), no backend, and no external services or databases.

This PR adds AGENTS.md documenting how to build/run the app and, importantly, a ## Cursor Cloud specific instructions section clarifying that the app cannot be built, run, linted, or tested on the Linux cloud VM — iOS development requires macOS + Xcode + the iOS SDK/Simulator.

Environment findings

• SDKROOT = iphoneos, IPHONEOS_DEPLOYMENT_TARGET = 10.3, SWIFT_VERSION = 3.0.
• The cloud VM is Ubuntu 24.04 (Linux); no swift/swiftc/xcodebuild present, and download.swift.org is unreachable from the VM.
• No dependencies to install → the startup update script is a no-op.
• No test target defined → no automated tests exist.

Changes

• Add AGENTS.md with build/run instructions and Cursor Cloud specific caveats.