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/KordantSpamShieldExtension/KordantSpamShieldExtension.entitlements

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>com.apple.developer.callkit</key>
+	<true/>
+	<key>com.apple.security.application-groups</key>
+	<array>
+		<string>group.com.frenocorp.kordant</string>
+	</array>
+</dict>
+</plist>