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>

GiteaIssueShare/GiteaIssueShare.entitlements

@@ -2,10 +2,6 @@
 <!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.security.application-groups</key>
-    <array>
-        <string>group.com.treytartt.GiteaIssue</string>
-    </array>
     <key>keychain-access-groups</key>
     <array>
         <string>$(AppIdentifierPrefix)com.treytartt.GiteaIssue</string>