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/security.md

@@ -0,0 +1,33 @@
+---
+paths:
+  - "**/*.swift"
+  - "**/Package.swift"
+---
+# Swift Security
+
+> This file extends [common/security.md](../common/security.md) with Swift specific content.
+
+## Secret Management
+
+- Use **Keychain Services** for sensitive data (tokens, passwords, keys) — never `UserDefaults`
+- Use environment variables or `.xcconfig` files for build-time secrets
+- Never hardcode secrets in source — decompilation tools extract them trivially
+
+```swift
+let apiKey = ProcessInfo.processInfo.environment["API_KEY"]
+guard let apiKey, !apiKey.isEmpty else {
+    fatalError("API_KEY not configured")
+}
+```
+
+## Transport Security
+
+- App Transport Security (ATS) is enforced by default — do not disable it
+- Use certificate pinning for critical endpoints
+- Validate all server certificates
+
+## Input Validation
+
+- Sanitize all user input before display to prevent injection
+- Use `URL(string:)` with validation rather than force-unwrapping
+- Validate data from external sources (APIs, deep links, pasteboard) before processing