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/testing.md

@@ -0,0 +1,45 @@
+---
+paths:
+  - "**/*.swift"
+  - "**/Package.swift"
+---
+# Swift Testing
+
+> This file extends [common/testing.md](../common/testing.md) with Swift specific content.
+
+## Framework
+
+Use **Swift Testing** (`import Testing`) for new tests. Use `@Test` and `#expect`:
+
+```swift
+@Test("User creation validates email")
+func userCreationValidatesEmail() throws {
+    #expect(throws: ValidationError.invalidEmail) {
+        try User(email: "not-an-email")
+    }
+}
+```
+
+## Test Isolation
+
+Each test gets a fresh instance — set up in `init`, tear down in `deinit`. No shared mutable state between tests.
+
+## Parameterized Tests
+
+```swift
+@Test("Validates formats", arguments: ["json", "xml", "csv"])
+func validatesFormat(format: String) throws {
+    let parser = try Parser(format: format)
+    #expect(parser.isValid)
+}
+```
+
+## Coverage
+
+```bash
+swift test --enable-code-coverage
+```
+
+## Reference
+
+See skill: `swift-protocol-di-testing` for protocol-based dependency injection and mock patterns with Swift Testing.