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/milestone_state.go

@@ -54,6 +54,6 @@ $ tea milestone %[1]s v1.0 v1.1`, verb), Annotations: map[string]string{"tea.arg
 		return errors.Join(append(failures, writeBatchJSON(a, cmd, len(selected), results))...)
 	}}
 	cmd.Flags().BoolVar(&byName, "by-name", false, "Match numeric arguments as milestone titles rather than IDs")
-	cmd.ValidArgsFunction = a.completeMilestoneArgs(true)
+	cmd.ValidArgsFunction = a.completeMilestones()
 	return cmd
 }