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/AnalyticsService.swift

@@ -18,10 +18,10 @@ enum Analytics {
                 let (_, response) = try await URLSession.shared.data(for: request)
                 let status = (response as? HTTPURLResponse)?.statusCode ?? 0
                 if status < 200 || status > 299 {
-                    print("[Analytics] \(event) → HTTP \(status)")
+                    Log.analytics.error("\(event, privacy: .public) → HTTP \(status, privacy: .public)")
                 }
             } catch {
-                print("[Analytics] \(event) failed: \(error.localizedDescription)")
+                Log.analytics.error("\(event, privacy: .public) failed: \(error.localizedDescription, privacy: .public)")
             }
         }
     }