Use when the user asks to "pick which social channels to run", "should we be on X platform or 小红书", or "plan our organic social channel portfolio"; produces an audience/objective-first portfolio with capability/access matrix, cadence-budget reality check, declared ECHO operating profile, boundary routing, and proposed-state registry events. Not for recording canonical channel facts — use channel-registry. 社媒渠道选择/渠道组合规划/平台能力矩阵/自然社媒