feat(skills): add swift-concurrency-6-2 skill for Approachable Concurrency

Add skill covering Swift 6.2 Approachable Concurrency model including
single-threaded defaults, @concurrent for explicit background offloading,
isolated conformances, and MainActor default inference mode.

skills/swift-concurrency-6-2/SKILL.md

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+---
+name: swift-concurrency-6-2
+description: Swift 6.2 Approachable Concurrency — single-threaded by default, @concurrent for explicit background offloading, isolated conformances for main actor types.
+---
+
+# Swift 6.2 Approachable Concurrency
+
+Patterns for adopting Swift 6.2's concurrency model where code runs single-threaded by default and concurrency is introduced explicitly. Eliminates common data-race errors without sacrificing performance.
+
+## When to Activate
+
+- Migrating Swift 5.x or 6.0/6.1 projects to Swift 6.2
+- Resolving data-race safety compiler errors
+- Designing MainActor-based app architecture
+- Offloading CPU-intensive work to background threads
+- Implementing protocol conformances on MainActor-isolated types
+- Enabling Approachable Concurrency build settings in Xcode 26
+
+## Core Problem: Implicit Background Offloading
+
+In Swift 6.1 and earlier, async functions could be implicitly offloaded to background threads, causing data-race errors even in seemingly safe code:
+
+```swift
+// Swift 6.1: ERROR
+@MainActor
+final class StickerModel {
+    let photoProcessor = PhotoProcessor()
+
+    func extractSticker(_ item: PhotosPickerItem) async throws -> Sticker? {
+        guard let data = try await item.loadTransferable(type: Data.self) else { return nil }
+
+        // Error: Sending 'self.photoProcessor' risks causing data races
+        return await photoProcessor.extractSticker(data: data, with: item.itemIdentifier)
+    }
+}
+```
+
+Swift 6.2 fixes this: async functions stay on the calling actor by default.
+
+```swift
+// Swift 6.2: OK — async stays on MainActor, no data race
+@MainActor
+final class StickerModel {
+    let photoProcessor = PhotoProcessor()
+
+    func extractSticker(_ item: PhotosPickerItem) async throws -> Sticker? {
+        guard let data = try await item.loadTransferable(type: Data.self) else { return nil }
+        return await photoProcessor.extractSticker(data: data, with: item.itemIdentifier)
+    }
+}
+```
+
+## Core Pattern — Isolated Conformances
+
+MainActor types can now conform to non-isolated protocols safely:
+
+```swift
+protocol Exportable {
+    func export()
+}
+
+// Swift 6.1: ERROR — crosses into main actor-isolated code
+// Swift 6.2: OK with isolated conformance
+extension StickerModel: @MainActor Exportable {
+    func export() {
+        photoProcessor.exportAsPNG()
+    }
+}
+```
+
+The compiler ensures the conformance is only used on the main actor:
+
+```swift
+// OK — ImageExporter is also @MainActor
+@MainActor
+struct ImageExporter {
+    var items: [any Exportable]
+
+    mutating func add(_ item: StickerModel) {
+        items.append(item)  // Safe: same actor isolation
+    }
+}
+
+// ERROR — nonisolated context can't use MainActor conformance
+nonisolated struct ImageExporter {
+    var items: [any Exportable]
+
+    mutating func add(_ item: StickerModel) {
+        items.append(item)  // Error: Main actor-isolated conformance cannot be used here
+    }
+}
+```
+
+## Core Pattern — Global and Static Variables
+
+Protect global/static state with MainActor:
+
+```swift
+// Swift 6.1: ERROR — non-Sendable type may have shared mutable state
+final class StickerLibrary {
+    static let shared: StickerLibrary = .init()  // Error
+}
+
+// Fix: Annotate with @MainActor
+@MainActor
+final class StickerLibrary {
+    static let shared: StickerLibrary = .init()  // OK
+}
+```
+
+### MainActor Default Inference Mode
+
+Swift 6.2 introduces a mode where MainActor is inferred by default — no manual annotations needed:
+
+```swift
+// With MainActor default inference enabled:
+final class StickerLibrary {
+    static let shared: StickerLibrary = .init()  // Implicitly @MainActor
+}
+
+final class StickerModel {
+    let photoProcessor: PhotoProcessor
+    var selection: [PhotosPickerItem]  // Implicitly @MainActor
+}
+
+extension StickerModel: Exportable {  // Implicitly @MainActor conformance
+    func export() {
+        photoProcessor.exportAsPNG()
+    }
+}
+```
+
+This mode is opt-in and recommended for apps, scripts, and other executable targets.
+
+## Core Pattern — @concurrent for Background Work
+
+When you need actual parallelism, explicitly offload with `@concurrent`:
+
+```swift
+nonisolated struct PhotoProcessor {
+    var cachedStickers: [String: Sticker]
+
+    func extractSticker(data: Data, with id: String) async -> Sticker {
+        if let sticker = cachedStickers[id] {
+            return sticker
+        }
+
+        let sticker = await Self.extractSubject(from: data)
+        cachedStickers[id] = sticker
+        return sticker
+    }
+
+    // Offload expensive work to concurrent thread pool
+    @concurrent
+    static func extractSubject(from data: Data) async -> Sticker { /* ... */ }
+}
+
+// Callers must await
+processedPhotos[item.id] = await PhotoProcessor().process(data: data)
+```
+
+To use `@concurrent`:
+1. Mark the containing type as `nonisolated`
+2. Add `@concurrent` to the function
+3. Add `async` if not already asynchronous
+4. Add `await` at call sites
+
+## Key Design Decisions
+
+| Decision | Rationale |
+|----------|-----------|
+| Single-threaded by default | Most natural code is data-race free; concurrency is opt-in |
+| Async stays on calling actor | Eliminates implicit offloading that caused data-race errors |
+| Isolated conformances | MainActor types can conform to protocols without unsafe workarounds |
+| `@concurrent` explicit opt-in | Background execution is a deliberate performance choice, not accidental |
+| MainActor default inference | Reduces boilerplate `@MainActor` annotations for app targets |
+| Opt-in adoption | Non-breaking migration path — enable features incrementally |
+
+## Migration Steps
+
+1. **Enable in Xcode**: Swift Compiler > Concurrency section in Build Settings
+2. **Enable in SPM**: Use `SwiftSettings` API in package manifest
+3. **Use migration tooling**: Automatic code changes via swift.org/migration
+4. **Start with MainActor defaults**: Enable inference mode for app targets
+5. **Add `@concurrent` where needed**: Profile first, then offload hot paths
+6. **Test thoroughly**: Data-race issues become compile-time errors
+
+## Best Practices
+
+- **Start on MainActor** — write single-threaded code first, optimize later
+- **Use `@concurrent` only for CPU-intensive work** — image processing, compression, complex computation
+- **Enable MainActor inference mode** for app targets that are mostly single-threaded
+- **Profile before offloading** — use Instruments to find actual bottlenecks
+- **Protect globals with MainActor** — global/static mutable state needs actor isolation
+- **Use isolated conformances** instead of `nonisolated` workarounds or `@Sendable` wrappers
+- **Migrate incrementally** — enable features one at a time in build settings
+
+## Anti-Patterns to Avoid
+
+- Applying `@concurrent` to every async function (most don't need background execution)
+- Using `nonisolated` to suppress compiler errors without understanding isolation
+- Keeping legacy `DispatchQueue` patterns when actors provide the same safety
+- Skipping `model.availability` checks in concurrency-related Foundation Models code
+- Fighting the compiler — if it reports a data race, the code has a real concurrency issue
+- Assuming all async code runs in the background (Swift 6.2 default: stays on calling actor)
+
+## When to Use
+
+- All new Swift 6.2+ projects (Approachable Concurrency is the recommended default)
+- Migrating existing apps from Swift 5.x or 6.0/6.1 concurrency
+- Resolving data-race safety compiler errors during Xcode 26 adoption
+- Building MainActor-centric app architectures (most UI apps)
+- Performance optimization — offloading specific heavy computations to background