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/BookmarkAssistant.swift

@@ -0,0 +1,81 @@
+import Foundation
+import Observation
+import FoundationModels
+
+/// Generation half of on-device RAG. Wraps a `LanguageModelSession` configured
+/// with `BookmarkSearchTool`, so every question is answered by Apple's
+/// on-device model grounded in the user's own bookmarks — no network, no
+/// third-party LLM. Mirrors the `ClaudeService` role, but fully on-device.
+@MainActor
+@Observable
+final class BookmarkAssistant {
+    enum Status: Equatable {
+        case checking
+        case ready
+        case unavailable(String)
+    }
+
+    private(set) var status: Status = .checking
+    private var session: LanguageModelSession?
+
+    init() { configure() }
+
+    private func configure() {
+        switch SystemLanguageModel.default.availability {
+        case .available:
+            session = LanguageModelSession(
+                tools: [BookmarkSearchTool()],
+                instructions: """
+                You are an assistant inside a bookmarks app. The user's saved bookmarks \
+                are your only source of truth. To answer any question, call the \
+                searchBookmarks tool to look things up — never rely on outside knowledge \
+                or invent links. Ground every answer in the returned bookmarks, refer to \
+                them by title, and include their URLs when relevant. If nothing relevant \
+                is found, say so plainly. Keep answers concise.
+                """
+            )
+            status = .ready
+            Log.assistant.info("On-device model available; session ready")
+        case .unavailable(let reason):
+            status = .unavailable(Self.message(for: reason))
+            Log.assistant.notice("On-device model unavailable: \(String(describing: reason), privacy: .public)")
+        }
+    }
+
+    /// Ask a question; the model retrieves from Spotlight via the tool as needed.
+    func ask(_ question: String) async throws -> String {
+        guard let session else {
+            Log.assistant.error("ask() with no session (model unavailable)")
+            throw AssistantError.unavailable
+        }
+        Log.assistant.info("Ask: \(question, privacy: .public)")
+        let start = Date()
+        do {
+            let answer = try await session.respond(to: question).content
+            let ms = Int(Date().timeIntervalSince(start) * 1000)
+            Log.assistant.info("Answered in \(ms, privacy: .public)ms, \(answer.count, privacy: .public) chars")
+            return answer
+        } catch {
+            Log.assistant.error("Ask failed: \(error.localizedDescription, privacy: .public)")
+            throw error
+        }
+    }
+
+    private static func message(for reason: SystemLanguageModel.Availability.UnavailableReason) -> String {
+        switch reason {
+        case .deviceNotEligible:
+            return "This device doesn't support Apple Intelligence."
+        case .appleIntelligenceNotEnabled:
+            return "Turn on Apple Intelligence in Settings to ask your bookmarks."
+        case .modelNotReady:
+            return "The on-device model is still downloading. Try again shortly."
+        @unknown default:
+            return "On-device intelligence isn't available right now."
+        }
+    }
+
+    enum AssistantError: LocalizedError {
+        case unavailable
+        var errorDescription: String? { "On-device intelligence is unavailable." }
+    }
+}