Add AGENTS.md with Cursor Cloud development environment setup

[bisonbet]

Summary

Adds AGENTS.md with Cursor Cloud-specific development instructions for this iOS/watchOS Xcode project.

What's in AGENTS.md

• Platform constraints: Documents that this project requires macOS + Xcode 15+ for building, running the simulator, and executing tests — these cannot be done on a Linux Cloud Agent VM.
• Available Linux tools: SwiftLint for linting (148 Swift files, ~9,000 pre-existing violations) and swiftc -parse for syntax validation.
• Environment setup: Swift 6.3.x via Swiftly and SwiftLint 0.58.x, with required environment variables for SourceKit.
• Running commands: How to run SwiftLint and Swift syntax checks from the Linux VM.
• Project structure: Quick reference to key paths (Xcode project, app source, tests, frameworks).

Environment Verification

SwiftLint successfully linted all 148 Swift files:

swiftlint_output.log

Why

Future Cloud Agents need to know that this is an iOS-only project with macOS/Xcode build requirements, and what development tasks they can perform on a Linux VM (linting, syntax checking, code review).

_{To show artifacts inline, enable in settings.}