feat(skills): add swift-actor-persistence skill

Thread-safe data persistence patterns using Swift actors with
in-memory cache and file-backed storage.

skills/swift-actor-persistence/SKILL.md

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+---
+name: swift-actor-persistence
+description: Thread-safe data persistence in Swift using actors — in-memory cache with file-backed storage, eliminating data races by design.
+---
+
+# Swift Actors for Thread-Safe Persistence
+
+Patterns for building thread-safe data persistence layers using Swift actors. Combines in-memory caching with file-backed storage, leveraging the actor model to eliminate data races at compile time.
+
+## When to Activate
+
+- Building a data persistence layer in Swift 5.5+
+- Need thread-safe access to shared mutable state
+- Want to eliminate manual synchronization (locks, DispatchQueues)
+- Building offline-first apps with local storage
+
+## Core Pattern
+
+### Actor-Based Repository
+
+The actor model guarantees serialized access — no data races, enforced by the compiler.
+
+```swift
+public actor LocalRepository<T: Codable & Identifiable> where T.ID == String {
+    private var cache: [String: T] = [:]
+    private let fileURL: URL
+
+    public init(directory: URL = .documentsDirectory, filename: String = "data.json") {
+        self.fileURL = directory.appendingPathComponent(filename)
+        // Synchronous load during init (actor isolation not yet active)
+        self.cache = Self.loadSynchronously(from: fileURL)
+    }
+
+    // MARK: - Public API
+
+    public func save(_ item: T) throws {
+        cache[item.id] = item
+        try persistToFile()
+    }
+
+    public func delete(_ id: String) throws {
+        cache[id] = nil
+        try persistToFile()
+    }
+
+    public func find(by id: String) -> T? {
+        cache[id]
+    }
+
+    public func loadAll() -> [T] {
+        Array(cache.values)
+    }
+
+    // MARK: - Private
+
+    private func persistToFile() throws {
+        let data = try JSONEncoder().encode(Array(cache.values))
+        try data.write(to: fileURL, options: .atomic)
+    }
+
+    private static func loadSynchronously(from url: URL) -> [String: T] {
+        guard let data = try? Data(contentsOf: url),
+              let items = try? JSONDecoder().decode([T].self, from: data) else {
+            return [:]
+        }
+        return Dictionary(uniqueKeysWithValues: items.map { ($0.id, $0) })
+    }
+}
+```
+
+### Usage
+
+All calls are automatically async due to actor isolation:
+
+```swift
+let repository = LocalRepository<Question>()
+
+// Read — fast O(1) lookup from in-memory cache
+let question = await repository.find(by: "q-001")
+let allQuestions = await repository.loadAll()
+
+// Write — updates cache and persists to file atomically
+try await repository.save(newQuestion)
+try await repository.delete("q-001")
+```
+
+### Combining with @Observable ViewModel
+
+```swift
+@Observable
+final class QuestionListViewModel {
+    private(set) var questions: [Question] = []
+    private let repository: LocalRepository<Question>
+
+    init(repository: LocalRepository<Question> = LocalRepository()) {
+        self.repository = repository
+    }
+
+    func load() async {
+        questions = await repository.loadAll()
+    }
+
+    func add(_ question: Question) async throws {
+        try await repository.save(question)
+        questions = await repository.loadAll()
+    }
+}
+```
+
+## Key Design Decisions
+
+| Decision | Rationale |
+|----------|-----------|
+| Actor (not class + lock) | Compiler-enforced thread safety, no manual synchronization |
+| In-memory cache + file persistence | Fast reads from cache, durable writes to disk |
+| Synchronous init loading | Avoids async initialization complexity |
+| Dictionary keyed by ID | O(1) lookups by identifier |
+| Generic over `Codable & Identifiable` | Reusable across any model type |
+| Atomic file writes (`.atomic`) | Prevents partial writes on crash |
+
+## Best Practices
+
+- **Use `Sendable` types** for all data crossing actor boundaries
+- **Keep the actor's public API minimal** — only expose domain operations, not persistence details
+- **Use `.atomic` writes** to prevent data corruption if the app crashes mid-write
+- **Load synchronously in `init`** — async initializers add complexity with minimal benefit for local files
+- **Combine with `@Observable`** ViewModels for reactive UI updates
+
+## Anti-Patterns to Avoid
+
+- Using `DispatchQueue` or `NSLock` instead of actors for new Swift concurrency code
+- Exposing the internal cache dictionary to external callers
+- Making the file URL configurable without validation
+- Forgetting that all actor method calls are `await` — callers must handle async context
+- Using `nonisolated` to bypass actor isolation (defeats the purpose)
+
+## When to Use
+
+- Local data storage in iOS/macOS apps (user data, settings, cached content)
+- Offline-first architectures that sync to a server later
+- Any shared mutable state that multiple parts of the app access concurrently
+- Replacing legacy `DispatchQueue`-based thread safety with modern Swift concurrency