Add AGENTS.md with Cursor Cloud development instructions

AGENTS.md

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+# CheckList - iOS App
+
+## Cursor Cloud specific instructions
+
+### Platform requirement
+This is a **native iOS app** built with Swift 3.0 and Xcode 8.3.2. It **cannot be built or run on Linux**. Building and running requires **macOS with Xcode 8.3+** (or a later version with Swift migration).
+
+### Project overview
+- **Language:** Swift 3.0 (`.xcodeproj` sets `SWIFT_VERSION = 3.0`)
+- **Target:** iOS 10.3 (iPhone + iPad)
+- **UI:** UIKit with Interface Builder Storyboards
+- **Data:** Core Data (`NSPersistentContainer` in `AppDelegate.swift`, model at `CheckList.xcdatamodeld` — currently has no entities defined)
+- **Dependencies:** None — no CocoaPods, Carthage, SPM, or external frameworks
+- **Tests:** No test targets exist in the project
+
+### What agents can do on Linux
+- Review and edit Swift source files (`AppDelegate.swift`, `ViewController.swift`)
+- Review and edit storyboard XML files (`Main.storyboard`, `LaunchScreen.storyboard`)
+- Validate the Xcode project file structure (`project.pbxproj`)
+- Swift syntax checking is **not possible** because the Swift toolchain cannot be downloaded in the restricted Cloud Agent network, and UIKit imports would fail on Linux regardless
+
+### Build & run (macOS only)
+```
+open CheckList.xcodeproj
+# In Xcode: select an iOS Simulator target, then Cmd+R to build and run
+```
+
+### Key caveats
+- The app is a skeleton — `ViewController.swift` is essentially empty
+- Core Data model has no entities defined yet
+- Swift 3.0 syntax will require migration if opened in modern Xcode (Xcode will offer automatic migration)