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/BookmarkSearchTool.swift

@@ -0,0 +1,37 @@
+import Foundation
+import FoundationModels
+
+/// The tool the on-device model calls to look things up in the user's library.
+/// It is the bridge between `LanguageModelSession` (generation) and the
+/// Spotlight index (retrieval) — together they form on-device RAG over the
+/// private bookmark corpus.
+struct BookmarkSearchTool: Tool {
+    let name = "searchBookmarks"
+    let description = """
+    Search the user's saved bookmarks. Returns the most relevant bookmarks with \
+    their title, website, description, and URL. Call this for any question about \
+    what the user has saved, read, or wants to find again.
+    """
+
+    @Generable
+    struct Arguments {
+        @Guide(description: "Keywords or a short phrase describing what to look for in the saved bookmarks")
+        var query: String
+    }
+
+    func call(arguments: Arguments) async throws -> String {
+        Log.tool.info("searchBookmarks invoked query=\(arguments.query, privacy: .public)")
+        let hits = await SpotlightBookmarkSearch.run(query: arguments.query, limit: 8)
+        Log.tool.info("searchBookmarks returned \(hits.count, privacy: .public) bookmarks")
+        guard !hits.isEmpty else {
+            return "No bookmarks matched \"\(arguments.query)\"."
+        }
+        return hits.enumerated().map { idx, b in
+            var line = "\(idx + 1). \(b.title)"
+            if !b.host.isEmpty { line += " — \(b.host)" }
+            if !b.description.isEmpty { line += "\n   \(b.description)" }
+            if !b.url.isEmpty { line += "\n   \(b.url)" }
+            return line
+        }.joined(separator: "\n")
+    }
+}