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.

oAI/ViewModels/ChatViewModel.swift

@@ -2294,7 +2294,9 @@ Don't narrate future actions ("Let me...") - just use the tools.
         alert.messageText = "Restore unsaved conversation?"
         alert.informativeText = "Confab didn't close properly last time. Would you like to restore the conversation you were working on?"
         alert.addButton(withTitle: "Restore")
-        alert.addButton(withTitle: "Discard")
+        let discardButton = alert.addButton(withTitle: "Discard")
+        discardButton.keyEquivalent = "d"
+        discardButton.keyEquivalentModifierMask = .command
 
         if alert.runModal() == .alertFirstButtonReturn {
             messages = draft.messages