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_issues.go

@@ -40,7 +40,7 @@ $ tea milestone issue list v1.0 --state all --kind issues`, Annotations: map[str
 	enumFlag(list, &opts.State, "state", "s", "open", "Filter by state", "open", "closed", "all")
 	enumFlag(list, &opts.Kind, "kind", "", "all", "List issues, pull requests or both", "issues", "pulls", "all")
 	list.Flags().BoolVar(&byName, "by-name", false, "Match a numeric argument as a milestone title rather than an ID")
-	list.ValidArgsFunction = a.completeMilestoneArgs(false)
+	list.ValidArgsFunction = firstArg(a.completeMilestones())
 	group.AddCommand(list)
 	for _, verb := range []string{"add", "remove"} {
 		var byName bool