Fix Git Sync race between startup pull and auto-sync export

syncOnStartup() (pull+import, fired at launch) and autoSync() (export+push,
debounced off chat activity) ran as fully independent, uncoordinated Tasks
with no mutual exclusion. A user launching the app and chatting right away
could hit autoSync's export mid-pull, leaving a freshly-written untracked
file that the pull then refuses to merge over — the same failure class as
the earlier folders.json bug, now much more likely to surface widely since
folders.json/notes.json are brand new for every existing sync repo.

Adds a shared isSyncing guard across all three entry points (syncOnStartup
skips if busy, autoSync waits for a clear slot, syncNow throws
.syncInProgress) and moves Sync Now's pull/import/export/push orchestration
out of SettingsView into GitSyncService.syncNow(), where the guard can
actually protect it.

oAI/Models/SyncModels.swift

@@ -39,6 +39,7 @@ enum SyncError: LocalizedError {
     case repoNotCloned
     case secretsDetected([String])
     case parseError(String)
+    case syncInProgress
 
     var errorDescription: String? {
         switch self {
@@ -56,6 +57,8 @@ enum SyncError: LocalizedError {
             return "Secrets detected in conversations: \(secrets.joined(separator: ", ")). Remove before syncing."
         case .parseError(let message):
             return "Failed to parse conversation: \(message)"
+        case .syncInProgress:
+            return "A sync is already in progress. Try again in a moment."
         }
     }
 }