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>

project.yml

@@ -89,6 +89,7 @@ targets:
       - path: Marks/Models/ServerConfig.swift
       - path: Marks/Services/LinkdingAPI.swift
       - path: Marks/Services/MarksAuth.swift
+      - path: Marks/Services/Log.swift
     settings:
       base:
         PRODUCT_BUNDLE_IDENTIFIER: com.magicive.marks.ShareExtension