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/coding-style.md

@@ -0,0 +1,47 @@
+---
+paths:
+  - "**/*.swift"
+  - "**/Package.swift"
+---
+# Swift Coding Style
+
+> This file extends [common/coding-style.md](../common/coding-style.md) with Swift specific content.
+
+## Formatting
+
+- **SwiftFormat** for auto-formatting, **SwiftLint** for style enforcement
+- `swift-format` is bundled with Xcode 16+ as an alternative
+
+## Immutability
+
+- Prefer `let` over `var` — define everything as `let` and only change to `var` if the compiler requires it
+- Use `struct` with value semantics by default; use `class` only when identity or reference semantics are needed
+
+## Naming
+
+Follow [Apple API Design Guidelines](https://www.swift.org/documentation/api-design-guidelines/):
+
+- Clarity at the point of use — omit needless words
+- Name methods and properties for their roles, not their types
+- Use `static let` for constants over global constants
+
+## Error Handling
+
+Use typed throws (Swift 6+) and pattern matching:
+
+```swift
+func load(id: String) throws(LoadError) -> Item {
+    guard let data = try? read(from: path) else {
+        throw .fileNotFound(id)
+    }
+    return try decode(data)
+}
+```
+
+## Concurrency
+
+Enable Swift 6 strict concurrency checking. Prefer:
+
+- `Sendable` value types for data crossing isolation boundaries
+- Actors for shared mutable state
+- Structured concurrency (`async let`, `TaskGroup`) over unstructured `Task {}`