feat: integrate KordantSpamShieldExtension target and complete App Review compliance (Task 28)

- Add KordantSpamShieldExtension target to project.yml with proper
  app-extension type, bundle identifier, and deployment target
- Create CallKit + App Group entitlements for SpamShield extension
- Move SpamDirectoryService to Sources/Shared for cross-target access
- Update app-review-checklist with 5 new technical items (total: 121)
- Update rejection-risk-mitigation with extension build integration
- Add SpamShield extension details to reviewer notes
- Mark Task 24 (push deep links) and Task 28 as complete

iOS/project.yml

@@ -52,6 +52,7 @@ targets:
         INFOPLIST_FILE: Info.plist
     dependencies:
       - target: KordantWidgets
+      - target: KordantSpamShieldExtension
       - package: Collections
         product: Collections
       - package: Algorithms
@@ -101,6 +102,25 @@ targets:
         CODE_SIGN_ENTITLEMENTS: KordantWidgets/KordantWidgets.entitlements
         SWIFT_VERSION: "5.9"
 
+  KordantSpamShieldExtension:
+    type: app-extension
+    platform: iOS
+    deploymentTarget: "17.0"
+    sources:
+      - path: KordantSpamShieldExtension
+        includes:
+          - "**/*.swift"
+          - "PrivacyInfo.xcprivacy"
+      - path: Sources/Shared
+    settings:
+      base:
+        PRODUCT_BUNDLE_IDENTIFIER: com.frenocorp.kordant.spamshield
+        PRODUCT_NAME: KordantSpamShieldExtension
+        SKIP_INSTALL: YES
+        INFOPLIST_FILE: KordantSpamShieldExtension/Info.plist
+        CODE_SIGN_ENTITLEMENTS: KordantSpamShieldExtension/KordantSpamShieldExtension.entitlements
+        SWIFT_VERSION: "5.9"
+
   KordantUITests:
     type: bundle.ui-testing
     platform: iOS
@@ -120,6 +140,7 @@ schemes:
       targets:
         Kordant: all
         KordantWidgets: all
+        KordantSpamShieldExtension: all
         KordantTests: [test]
         KordantUITests: [test]
     run: