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/app/app_test.go

@@ -418,23 +418,9 @@ const (
 	shortsToFix = `
 `
 	examplesToWrite = `
-	issue attachment delete
-	issue attachment download
-	issue attachment edit
-	issue attachment list
-	issue attachment upload
-	issue attachment view
-	pr attachment delete
-	pr attachment download
-	pr attachment edit
-	pr attachment list
-	pr attachment upload
-	pr attachment view
 	search issue
 	search pr
 `
 	longsToWrite = `
-	issue attachment
-	pr attachment
 `
 )