gh project item-add suppresses success output when stdout is not a TTY #14055

Describe the bug

gh project item-add exits successfully but prints no confirmation when stdout is not a TTY. This makes successful automation and agent-driven calls indistinguishable from a command that produced no useful result.

The command currently gates its success message on IsStdoutTTY():

func printResults(config addItemConfig, item queries.ProjectItem) error {
	if !config.io.IsStdoutTTY() {
		return nil
	}

	_, err := fmt.Fprintf(config.io.Out, "Added item\n")
	return err
}

Steps to reproduce

Run gh project item-add with stdout redirected or through a non-TTY command runner:

gh project item-add <project-number> --owner <owner> --url <issue-url> >output.txt

The item is added and the command exits 0, but output.txt is empty.

Expected behavior

A successful non-TTY invocation should emit a terse, stable confirmation, ideally including the added project item identifier. Structured output via --format json remains available when callers need the full item.

Impact

This causes agents and automation to retry an already-successful mutation because they cannot observe success. In repeated Copilot CLI evaluations, the same one-line task varied from 2 tool calls to 9 tool calls; the expensive runs repeatedly invoked gh project item-add after the first call had already succeeded.