fix(checkpoint): short-circuit cancelled contexts in backfillTranscript

Copilot review finding on #1824: backfillTranscript lacked the ctx.Err()
guard its two backfill siblings have, so a cancelled context could still
reach the (now potentially fetching) base resolution.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: peyton-alt

cmd/entire/cli/checkpoint/refs_store.go

@@ -209,6 +209,9 @@ func (s *gitRefsStore) writeSession(ctx context.Context, opts WriteOptions) erro
 }
 
 func (s *gitRefsStore) backfillTranscript(ctx context.Context, opts UpdateOptions) error {
+	if err := ctx.Err(); err != nil {
+		return err //nolint:wrapcheck // Propagating context cancellation
+	}
 	if opts.CheckpointID.IsEmpty() {
 		return errors.New("invalid update options: checkpoint ID is required")
 	}