feat(attachment): download attachments under their own name

issue attachment download (and pr attachment download) required
--output-file. It now writes ./<name> like gh release download, refusing
to replace an existing file unless --clobber (or --skip-existing to keep
it), with -O/--output and -D/--dir from the shared download flags.
attachment list shows ID NAME SIZE DOWNLOADS CREATED with readable
sizes, attachment view is a view, and upload, rename and delete report
✓ lines; delete names the attachment in its confirmation. The commands
accept issue and pull request URLs and have help text with examples.

Signed-off-by: bircni <bircni@icloud.com>

internal/services/issue_attachments.go

@@ -65,11 +65,13 @@ func (s Issues) UploadAttachment(ctx context.Context, number int64, name string,
 	return IssueAttachment(asset), err
 }
 
-// DownloadAttachment opens an attachment stream scoped to its instance and issue.
-func (s Issues) DownloadAttachment(ctx context.Context, number, id int64) (io.ReadCloser, error) {
+// DownloadAttachment opens an attachment stream scoped to its instance and issue, and returns the
+// attachment's metadata.
+func (s Issues) DownloadAttachment(ctx context.Context, number, id int64) (IssueAttachment, io.ReadCloser, error) {
 	attachment, err := s.attachment(ctx, number, id)
 	if err != nil {
-		return nil, err
+		return IssueAttachment{}, nil, err
 	}
-	return downloadAttachment(ctx, s.Client, attachment)
+	body, err := downloadAttachment(ctx, s.Client, attachment)
+	return IssueAttachment(assetFrom(attachment)), body, err
 }