refactor(completion): name path arguments explicitly and share firstArg

The six commands that take a file or directory argument set their
completer directly instead of a Use-line parser wrapping every
command, the first-argument-only wrappers become one firstArg helper,
and only the bash completion help keeps a custom text.

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

internal/app/issue_attachments.go

@@ -90,7 +90,7 @@ are addressed by the %s number and the attachment ID that attachment list shows.
 
 	var uploadName string
 	upload := &cobra.Command{Use: "upload {<number> | <url>} <file>", Short: "Attach a file", Example: fmt.Sprintf(`$ tea %[1]s attachment upload %[2]s crash.log
-$ tea %[1]s attachment upload %[2]s screenshot.png --name before.png`, words.group, words.example), Annotations: annotations, Args: argsExact(2), ValidArgsFunction: completeFirst, RunE: func(cmd *cobra.Command, args []string) error {
+$ tea %[1]s attachment upload %[2]s screenshot.png --name before.png`, words.group, words.example), Annotations: annotations, Args: argsExact(2), ValidArgsFunction: pathArgs(1, false, completeFiles, completeFirst), RunE: func(cmd *cobra.Command, args []string) error {
 		name := uploadName
 		if cmd.Flags().Changed("name") && strings.TrimSpace(name) == "" {
 			return clierr.Wrap(clierr.Usage, errors.New("--name cannot be empty"))