feat(skills): add swift-protocol-di-testing skill

Protocol-based dependency injection patterns for testable Swift code
with Swift Testing framework examples.

skills/swift-protocol-di-testing/SKILL.md

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+---
+name: swift-protocol-di-testing
+description: Protocol-based dependency injection for testable Swift code — mock file system, network, and external APIs using focused protocols and Swift Testing.
+---
+
+# Swift Protocol-Based Dependency Injection for Testing
+
+Patterns for making Swift code testable by abstracting external dependencies (file system, network, iCloud) behind small, focused protocols. Enables deterministic tests without I/O.
+
+## When to Activate
+
+- Writing Swift code that accesses file system, network, or external APIs
+- Need to test error handling paths without triggering real failures
+- Building modules that work across environments (app, test, SwiftUI preview)
+- Designing testable architecture with Swift concurrency (actors, Sendable)
+
+## Core Pattern
+
+### 1. Define Small, Focused Protocols
+
+Each protocol handles exactly one external concern.
+
+```swift
+// File system access
+public protocol FileSystemProviding: Sendable {
+    func containerURL(for purpose: Purpose) -> URL?
+}
+
+// File read/write operations
+public protocol FileAccessorProviding: Sendable {
+    func read(from url: URL) throws -> Data
+    func write(_ data: Data, to url: URL) throws
+    func fileExists(at url: URL) -> Bool
+}
+
+// Bookmark storage (e.g., for sandboxed apps)
+public protocol BookmarkStorageProviding: Sendable {
+    func saveBookmark(_ data: Data, for key: String) throws
+    func loadBookmark(for key: String) throws -> Data?
+}
+```
+
+### 2. Create Default (Production) Implementations
+
+```swift
+public struct DefaultFileSystemProvider: FileSystemProviding {
+    public init() {}
+
+    public func containerURL(for purpose: Purpose) -> URL? {
+        FileManager.default.url(forUbiquityContainerIdentifier: nil)
+    }
+}
+
+public struct DefaultFileAccessor: FileAccessorProviding {
+    public init() {}
+
+    public func read(from url: URL) throws -> Data {
+        try Data(contentsOf: url)
+    }
+
+    public func write(_ data: Data, to url: URL) throws {
+        try data.write(to: url, options: .atomic)
+    }
+
+    public func fileExists(at url: URL) -> Bool {
+        FileManager.default.fileExists(atPath: url.path)
+    }
+}
+```
+
+### 3. Create Mock Implementations for Testing
+
+```swift
+public final class MockFileAccessor: FileAccessorProviding, @unchecked Sendable {
+    public var files: [URL: Data] = [:]
+    public var readError: Error?
+    public var writeError: Error?
+
+    public init() {}
+
+    public func read(from url: URL) throws -> Data {
+        if let error = readError { throw error }
+        guard let data = files[url] else {
+            throw CocoaError(.fileReadNoSuchFile)
+        }
+        return data
+    }
+
+    public func write(_ data: Data, to url: URL) throws {
+        if let error = writeError { throw error }
+        files[url] = data
+    }
+
+    public func fileExists(at url: URL) -> Bool {
+        files[url] != nil
+    }
+}
+```
+
+### 4. Inject Dependencies with Default Parameters
+
+Production code uses defaults; tests inject mocks.
+
+```swift
+public actor SyncManager {
+    private let fileSystem: FileSystemProviding
+    private let fileAccessor: FileAccessorProviding
+
+    public init(
+        fileSystem: FileSystemProviding = DefaultFileSystemProvider(),
+        fileAccessor: FileAccessorProviding = DefaultFileAccessor()
+    ) {
+        self.fileSystem = fileSystem
+        self.fileAccessor = fileAccessor
+    }
+
+    public func sync() async throws {
+        guard let containerURL = fileSystem.containerURL(for: .sync) else {
+            throw SyncError.containerNotAvailable
+        }
+        let data = try fileAccessor.read(
+            from: containerURL.appendingPathComponent("data.json")
+        )
+        // Process data...
+    }
+}
+```
+
+### 5. Write Tests with Swift Testing
+
+```swift
+import Testing
+
+@Test("Sync manager handles missing container")
+func testMissingContainer() async {
+    let mockFileSystem = MockFileSystemProvider(containerURL: nil)
+    let manager = SyncManager(fileSystem: mockFileSystem)
+
+    await #expect(throws: SyncError.containerNotAvailable) {
+        try await manager.sync()
+    }
+}
+
+@Test("Sync manager reads data correctly")
+func testReadData() async throws {
+    let mockFileAccessor = MockFileAccessor()
+    mockFileAccessor.files[testURL] = testData
+
+    let manager = SyncManager(fileAccessor: mockFileAccessor)
+    let result = try await manager.loadData()
+
+    #expect(result == expectedData)
+}
+
+@Test("Sync manager handles read errors gracefully")
+func testReadError() async {
+    let mockFileAccessor = MockFileAccessor()
+    mockFileAccessor.readError = CocoaError(.fileReadCorruptFile)
+
+    let manager = SyncManager(fileAccessor: mockFileAccessor)
+
+    await #expect(throws: SyncError.self) {
+        try await manager.sync()
+    }
+}
+```
+
+## Best Practices
+
+- **Single Responsibility**: Each protocol should handle one concern — don't create "god protocols" with many methods
+- **Sendable conformance**: Required when protocols are used across actor boundaries
+- **Default parameters**: Let production code use real implementations by default; only tests need to specify mocks
+- **Error simulation**: Design mocks with configurable error properties for testing failure paths
+- **Only mock boundaries**: Mock external dependencies (file system, network, APIs), not internal types
+
+## Anti-Patterns to Avoid
+
+- Creating a single large protocol that covers all external access
+- Mocking internal types that have no external dependencies
+- Using `#if DEBUG` conditionals instead of proper dependency injection
+- Forgetting `Sendable` conformance when used with actors
+- Over-engineering: if a type has no external dependencies, it doesn't need a protocol
+
+## When to Use
+
+- Any Swift code that touches file system, network, or external APIs
+- Testing error handling paths that are hard to trigger in real environments
+- Building modules that need to work in app, test, and SwiftUI preview contexts
+- Apps using Swift concurrency (actors, structured concurrency) that need testable architecture