Drop App Groups, share via keychain access group only

Apple's ASC API doesn't expose App Group registration cleanly, and the
keychain access group on its own (`$(AppIdentifierPrefix)<bundleId>`)
already gives the share extension and container app a shared store for
the base URL + token. The repo cache moves to per-process UserDefaults —
the extension fetches/caches it on first share if empty.

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

Shared/Settings.swift

@@ -1,8 +1,6 @@
 import Foundation
 
 enum AppSettings {
-    static let appGroup = "group.com.treytartt.GiteaIssue"
-
     private static let baseURLKey = "gitea.baseURL"
     private static let tokenKey = "gitea.token"
 
@@ -20,7 +18,7 @@ enum AppSettings {
         !token.isEmpty && URL(string: baseURL) != nil
     }
 
-    static var sharedDefaults: UserDefaults {
-        UserDefaults(suiteName: appGroup) ?? .standard
-    }
+    /// Process-local cache store. Keychain handles cross-process sharing;
+    /// the cached repo list lives in each process's standard defaults.
+    static var localDefaults: UserDefaults { .standard }
 }