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_edit.go

@@ -124,7 +124,7 @@ $ tea issue edit 12 --body-file notes.md`, Annotations: map[string]string{"tea.a
 	a.completeMetadataFlags(cmd)
 	_ = cmd.RegisterFlagCompletionFunc("milestone", a.completeMilestones())
 	_ = cmd.RegisterFlagCompletionFunc("ref", a.completeBranches())
-	cmd.ValidArgsFunction = a.completeIssueArgs("open", true)
+	cmd.ValidArgsFunction = a.completeIssues("open")
 	return cmd
 }