Revise Content Brief

Rewrite a content brief from a natural-language instruction (for example: 'add a section on pricing objections', 'cut the competitor comparison'). Records each pass as a version; iterate by calling it again, since each call starts from the current brief. Most useful while a piece waits at status generated_brief, where the brief still shapes what the article will say — call approve_brief once it reads right. It also works after the article exists, to correct the brief for a later regeneration; note that revising the brief does NOT rewrite an already-generated article, so use revise_content for that.