Add Xcode example project for build system integration

[obj-p]

Summary

• Adds examples/xcode/ — minimal SwiftUI framework managed by XcodeGen (project.yml), using Mint (Mintfile) for tool installation
• Same ToDo app (Item model + ToDoView with #Preview) as spm/ and bazel/ examples, duplicated for test isolation
• Framework target with supportedDestinations: [macOS, iOS], Swift 6.0 strict concurrency, macOS 14 / iOS 17 deployment targets
• .xcodeproj is git-ignored — regenerated from project.yml via xcodegen generate
• Updates bootstrap skill with Xcode example setup step (step 5)
• Updates examples/README.md to reflect the new example

Setup

brew install mint
cd examples/xcode
mint bootstrap        # installs XcodeGen from Mintfile
xcodegen generate     # creates ToDo.xcodeproj
xcodebuild build -project ToDo.xcodeproj -scheme ToDo -destination 'platform=macOS'

Test plan

• mint bootstrap installs XcodeGen 2.42.0
• xcodegen generate creates ToDo.xcodeproj from project.yml
• xcodebuild build succeeds for macOS destination
• xcodebuild build succeeds for iOS Simulator destination
• .swiftmodule produced in DerivedData for both platforms
• swift-format lint --strict passes on all new Swift files
• /bootstrap skill includes Xcode example step

Closes #16 (example project portion)

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