Attach a Gmail thread to the task [write]

Reads one Gmail thread and keeps it in the task's files as plain text at email/<subject>-<thread id>.md: subject, participants, and every message with sender, recipients, time and text; attachments are listed by name and never copied. Every session of the task reads it at its next turn, and the task story shows it as file.changed by the caller with detail.email. Attaching the same thread again writes a new version only when it changed (200 instead of 201). Nothing is sent from Gmail. Same access rules as the search; 400 invalid_thread for something that is not a thread id or a Gmail address ending in one; 404 thread_not_found.