feat: add Swift language-specific rules

Add 5 rule files for Swift following the established pattern used by
TypeScript, Python, and Go rule sets. Covers Swift 6 strict concurrency,
Swift Testing framework, protocol-oriented patterns, Keychain-based
secret management, and SwiftFormat/SwiftLint hooks.

rules/swift/patterns.md

@@ -0,0 +1,66 @@
+---
+paths:
+  - "**/*.swift"
+  - "**/Package.swift"
+---
+# Swift Patterns
+
+> This file extends [common/patterns.md](../common/patterns.md) with Swift specific content.
+
+## Protocol-Oriented Design
+
+Define small, focused protocols. Use protocol extensions for shared defaults:
+
+```swift
+protocol Repository: Sendable {
+    associatedtype Item: Identifiable & Sendable
+    func find(by id: Item.ID) async throws -> Item?
+    func save(_ item: Item) async throws
+}
+```
+
+## Value Types
+
+- Use structs for data transfer objects and models
+- Use enums with associated values to model distinct states:
+
+```swift
+enum LoadState<T: Sendable>: Sendable {
+    case idle
+    case loading
+    case loaded(T)
+    case failed(Error)
+}
+```
+
+## Actor Pattern
+
+Use actors for shared mutable state instead of locks or dispatch queues:
+
+```swift
+actor Cache<Key: Hashable & Sendable, Value: Sendable> {
+    private var storage: [Key: Value] = [:]
+
+    func get(_ key: Key) -> Value? { storage[key] }
+    func set(_ key: Key, value: Value) { storage[key] = value }
+}
+```
+
+## Dependency Injection
+
+Inject protocols with default parameters — production uses defaults, tests inject mocks:
+
+```swift
+struct UserService {
+    private let repository: any UserRepository
+
+    init(repository: any UserRepository = DefaultUserRepository()) {
+        self.repository = repository
+    }
+}
+```
+
+## References
+
+See skill: `swift-actor-persistence` for actor-based persistence patterns.
+See skill: `swift-protocol-di-testing` for protocol-based DI and testing.