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."