feat(ai): on-device RAG over bookmarks + Spotlight indexing + unified logging

- SpotlightIndexer: actually push BookmarkEntity into the Spotlight index via
  indexAppEntities (IndexedEntity conformance alone indexes nothing). Wired into
  every sync/mutation point. This is what lets Apple Intelligence answer
  free-form Siri questions grounded in the user's bookmarks.
- On-device RAG: BookmarkAssistant (LanguageModelSession) + BookmarkSearchTool
  + SpotlightBookmarkSearch (CSSearchQuery retrieval), surfaced via AskView with
  lightweight Markdown rendering of answers.
- Log: os.Logger facility (com.magicive.marks) replacing ad-hoc print(); shared
  with ShareExtension. ClaudeService now logs server error bodies on non-2xx.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Marks/Services/MarksAuth.swift

@@ -46,7 +46,7 @@ enum MarksAuth {
         let resp = try JSONDecoder().decode(Resp.self, from: data)
         UserDefaults.standard.set(resp.access_token, forKey: tokenKey)
         UserDefaults.standard.set(Date().addingTimeInterval(TimeInterval(resp.expires_in)), forKey: tokenExpiryKey)
-        print("[Auth] Registered \(id.prefix(8))…")
+        Log.auth.info("Registered device \(id.prefix(8), privacy: .public)…")
         return resp.access_token
     }
 }