Sync conversation notes via Git Sync, add discard shortcut to crash-recovery prompt

Notes files now export to notes/ + notes.json alongside conversations.json,
matching folders.json's manifest pattern: matched by conversation ID, never
overwrites notes a machine already has locally, same empty-state orphan-
cleanup safety guard as the existing conversation/folder sync code. Also
adds Cmd+D to the "Discard" button on the crash-recovery restore prompt.

Fixes two Swift 6 actor-isolation build warnings surfaced along the way:
ConversationNotesService and the String filename-sanitizing extension are
pure, state-free helpers called from nonisolated contexts (DatabaseService,
GitSyncService) but defaulted to @MainActor — marked nonisolated.

oAITests/ConversationNotesServiceTests.swift

@@ -51,6 +51,29 @@ struct ConversationNotesServiceTests {
         #expect(body == nil)
     }
 
+    @Test("readRaw returns the exact on-disk content, ID header included")
+    func readRawIncludesHeader() {
+        let id = UUID()
+        let filename = "test-notes-\(UUID().uuidString).md"
+        defer { ConversationNotesService.shared.delete(filename: filename) }
+
+        ConversationNotesService.shared.write(body: "User prefers dark roast coffee.", filename: filename, conversationId: id)
+        let raw = ConversationNotesService.shared.readRaw(filename: filename)
+
+        #expect(raw == "**ID**: `\(id.uuidString)`\n\nUser prefers dark roast coffee.")
+    }
+
+    @Test("writeRaw then readRaw round-trips content byte-for-byte, with no header wrapping added")
+    func writeRawReadRawRoundTrip() {
+        let filename = "test-notes-\(UUID().uuidString).md"
+        defer { ConversationNotesService.shared.delete(filename: filename) }
+        let content = "**ID**: `12345678-1234-1234-1234-123456789012`\n\nImported from another machine."
+
+        ConversationNotesService.shared.writeRaw(content: content, filename: filename)
+
+        #expect(ConversationNotesService.shared.readRaw(filename: filename) == content)
+    }
+
     @Test("stripIDHeader removes the embedded ID line and following blank line")
     func stripIDHeaderRemovesHeader() {
         let content = "**ID**: `12345678-1234-1234-1234-123456789012`\n\nActual notes content."

oAITests/GitSyncServiceTests.swift

@@ -232,4 +232,57 @@ struct GitSyncServiceTests {
         #expect(decoded.folders.first?.name == "Work")
         #expect(decoded.assignments[conversationId] == folderId)
     }
+
+    // MARK: - orphanedNoteFilenames
+
+    @Test("A note file still referenced by a current conversation is not orphaned")
+    func keepsNoteFilesForExistingConversations() {
+        let orphans = GitSyncService.orphanedNoteFilenames(currentFilenames: ["chat-a3f2.md"], existingFiles: ["chat-a3f2.md"])
+        #expect(orphans.isEmpty)
+    }
+
+    @Test("A note file no longer referenced by any current conversation is orphaned")
+    func flagsNoteFilesForDeletedConversations() {
+        let orphans = GitSyncService.orphanedNoteFilenames(
+            currentFilenames: ["kept-b1c2.md"],
+            existingFiles: ["kept-b1c2.md", "deleted-d3e4.md"]
+        )
+        #expect(orphans == ["deleted-d3e4.md"])
+    }
+
+    @Test("Non-markdown files in the notes directory are never flagged as orphans")
+    func nonMarkdownFilesAreNotFlaggedAsOrphans() {
+        let orphans = GitSyncService.orphanedNoteFilenames(currentFilenames: ["kept.md"], existingFiles: [".DS_Store"])
+        #expect(orphans.isEmpty)
+    }
+
+    @Test("Empty existing file list produces no orphans")
+    func emptyExistingNoteFilesProducesNoOrphans() {
+        #expect(GitSyncService.orphanedNoteFilenames(currentFilenames: ["a.md"], existingFiles: []).isEmpty)
+    }
+
+    @Test("Empty current filenames set never orphans existing note files, even when files exist")
+    func emptyCurrentNoteFilenamesNeverOrphansExistingFiles() {
+        // Same class of regression guard as emptyCurrentIdsNeverOrphansExistingFiles /
+        // emptyManifestNeverOrphansLocalFolders: an empty local-state snapshot must never be
+        // read as "delete everything in the sync repo."
+        let orphans = GitSyncService.orphanedNoteFilenames(currentFilenames: [], existingFiles: ["a.md", "b.md"])
+        #expect(orphans.isEmpty)
+    }
+
+    // MARK: - NotesSyncManifest round-trip
+
+    @Test("NotesSyncManifest round-trips through JSON encode/decode")
+    func notesSyncManifestRoundTrips() throws {
+        let conversationId = UUID().uuidString
+        let manifest = NotesSyncManifest(
+            notes: [conversationId: NotesSyncManifest.Entry(filename: "Chat-a3f2.md", enabled: true)]
+        )
+
+        let data = try JSONEncoder().encode(manifest)
+        let decoded = try JSONDecoder().decode(NotesSyncManifest.self, from: data)
+
+        #expect(decoded.notes[conversationId]?.filename == "Chat-a3f2.md")
+        #expect(decoded.notes[conversationId]?.enabled == true)
+    }
 }