Fix Git Sync wiping the entire repo when a fresh clone's DB is empty

exportAllConversations()'s orphan-cleanup treated "zero local
conversations" as "every synced conversation was deleted," so a
just-cloned repo on a new machine could get emptied and pushed before
the post-clone import ever ran. orphanedExportFilenames() now returns
no orphans when the local ID set is empty, and cloneRepository()
imports immediately after cloning to close the window entirely.

oAITests/GitSyncServiceTests.swift

@@ -151,4 +151,18 @@ struct GitSyncServiceTests {
     func emptyFileListProducesNoOrphans() {
         #expect(GitSyncService.orphanedExportFilenames(currentIds: [UUID().uuidString], files: []).isEmpty)
     }
+
+    @Test("Empty local conversation list never flags files as orphaned, even when files exist")
+    func emptyCurrentIdsNeverOrphansExistingFiles() {
+        // Regression test: this is the exact shape of the production bug. A fresh clone (or any
+        // moment where the local DB hasn't caught up yet) has zero local conversations, while the
+        // sync repo still has every previously-synced file on disk. Without the guard, all of them
+        // used to be flagged orphaned and deleted+pushed, wiping the whole sync repo.
+        let files = [
+            (filename: "a.md", markdown: makeExportMarkdown(id: UUID().uuidString)),
+            (filename: "b.md", markdown: makeExportMarkdown(id: UUID().uuidString)),
+        ]
+        let orphans = GitSyncService.orphanedExportFilenames(currentIds: [], files: files)
+        #expect(orphans.isEmpty)
+    }
 }