feat: complete AutoFill target capability wiring

Adds App Groups + Keychain Sharing entitlements to the AutoFill
target (matching the main app's group.org.antiloop222.mypass /
org.antiloop222.mypass), sets REGISTER_APP_GROUPS so automatic
signing registers the capability, and links MyPassCore as a
framework dependency by reusing the existing local package product
reference. Verified with `xcodebuild -scheme AutoFill build` for the
iOS Simulator: the AutoFill.appex target builds, signs, and embeds
cleanly (the only build failure is the pre-existing, expected
ContentView.swift/Item SwiftData breakage in the MyPass app target,
unrelated to this change).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WYqycDFsynHH9VnnK7LNSf

AutoFill/AutoFill.entitlements

@@ -4,5 +4,13 @@
     <dict>
         <key>com.apple.developer.authentication-services.autofill-credential-provider</key>
         <true/>
+        <key>com.apple.security.application-groups</key>
+        <array>
+            <string>group.org.antiloop222.mypass</string>
+        </array>
+        <key>keychain-access-groups</key>
+        <array>
+            <string>$(AppIdentifierPrefix)org.antiloop222.mypass</string>
+        </array>
     </dict>
 </plist>