WIP: local handoff implementation

Local work on the social.io handoff before merging the claude
worktree branch. Includes the full per-spec Sources/Core/Design
module (8 files), watchOS target under WatchApp/, Live Activity +
widget extension, entitlements, scheme, and asset catalog.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

swift/WatchApp/SocialIOWatchApp.swift

@@ -0,0 +1,14 @@
+import SwiftUI
+
+@main
+struct SocialIOWatchApp: App {
+    @State private var store = WatchInboxStore()
+
+    var body: some Scene {
+        WindowGroup {
+            NavigationStack {
+                WatchInboxView(store: store)
+            }
+        }
+    }
+}