fix(test): remove pre-existing issues.jsonl in TestExportOnlySync

setupGitRepo now creates an empty issues.jsonl for RepoContext,
which triggers 'JSONL changed since import' validation in tests
that set up their own .beads/ directory.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/sync_modes_test.go

@@ -681,6 +681,9 @@ func TestExportOnlySync(t *testing.T) {
 	beadsDir := filepath.Join(tmpDir, ".beads")
 	jsonlPath := filepath.Join(beadsDir, "issues.jsonl")
 
+	// Remove pre-existing issues.jsonl from setupGitRepo (we want a clean slate)
+	_ = os.Remove(jsonlPath)
+
 	// Setup: Create .beads directory
 	if err := os.MkdirAll(beadsDir, 0755); err != nil {
 		t.Fatalf("failed to create .beads dir: %v", err)