fix(sync): atomic export and force-push detection (bd-3bhl, bd-4hh5)

bd-3bhl: Add sync rollback on git commit failure
- Use exportToJSONLDeferred() instead of exportToJSONL() for atomic sync
- Call finalizeExport() only after git commit succeeds
- Rollback JSONL from git HEAD on commit failure
- Add rollbackJSONLFromGit() helper function
- Coverage: regular commit, sync branch, external beads repo paths

bd-4hh5: Fix false-positive force-push detection
- Use explicit refspec in CheckForcePush fetch
- +refs/heads/beads-sync:refs/remotes/origin/beads-sync
- Ensures tracking ref is always created/updated
- Fixes stale ref comparison causing false positives

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

cmd/bd/sync_git.go

@@ -492,6 +492,27 @@ func parseGitStatusForBeadsChanges(statusOutput string) bool {
 	return false
 }
 
+// rollbackJSONLFromGit restores the JSONL file from git HEAD after a failed commit.
+// This is part of the sync atomicity fix (GH#885/bd-3bhl): when git commit fails
+// after export, we restore the JSONL to its previous state so the working
+// directory stays consistent with the last successful sync.
+func rollbackJSONLFromGit(ctx context.Context, jsonlPath string) error {
+	// Check if the file is tracked by git
+	cmd := exec.CommandContext(ctx, "git", "ls-files", "--error-unmatch", jsonlPath)
+	if err := cmd.Run(); err != nil {
+		// File not tracked - nothing to restore
+		return nil
+	}
+
+	// Restore from HEAD
+	restoreCmd := exec.CommandContext(ctx, "git", "checkout", "HEAD", "--", jsonlPath) //nolint:gosec // G204: jsonlPath from internal beads.FindBeadsDir()
+	output, err := restoreCmd.CombinedOutput()
+	if err != nil {
+		return fmt.Errorf("git checkout failed: %w\n%s", err, output)
+	}
+	return nil
+}
+
 // getDefaultBranch returns the default branch name (main or master) for origin remote
 // Checks remote HEAD first, then falls back to checking if main/master exist
 func getDefaultBranch(ctx context.Context) string {