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/labels.go

@@ -302,7 +302,7 @@ func (a *application) labelImportCommand(scope labelScope) *cobra.Command {
 " ;description" and " ;exclusive", " ;archived" or both. tea label export writes this
 format, and Gitea's label templates use it. Existing labels are refused unless --force
 updates them.`, Example: `$ tea label import labels.txt
-$ tea label export --repo alice/old | tea label import - --repo alice/new`, Args: argsExact(1), RunE: func(cmd *cobra.Command, args []string) error {
+$ tea label export --repo alice/old | tea label import - --repo alice/new`, Args: argsExact(1), ValidArgsFunction: completeFiles, RunE: func(cmd *cobra.Command, args []string) error {
 		return a.importLabels(cmd, scope, args[0], force)
 	}}
 	cmd.Flags().BoolVarP(&force, "force", "f", false, "Update labels that exist already")