Task tool: omitting subagent_type now fails with the list of available agents

Previously, omitting subagent_type on a Task tool call silently fell back to a default agent. Now, unless the caller's agent set unambiguously implies one, the launch is rejected with an error that lists the available agents ("none" if there aren't any), reported as subagent_type_missing alongside a tengu_subagent_type_miss event carrying the count of available agents. The fork-specific refusal checks, for remote isolation and recursive fork, were factored into one shared checker so the same messages are reused when building that list.