feat(backthink): 自主思考 P2——结构化推送决策+渠道选择(design/20)

- proactive.go(新): parseProactive 从结论解析 [我决定:push|no_push][渠道:...]<正文>;
  兼容大小写/全半角冒号; 无标记回退(无/nothing=不推); 畸形/孤标记 fail-safe 不推
- context.go: guidancePrompt 引导结构化结尾+保留P1温暖引导
- push.go: PushRequest{Message,ChannelHint}; Pusher.Push 路由(auto→active优先/IM兜底,
  web→仅active, im→仅IM, 具体平台→IM+platform打标+缺IM回退默认记detail); buildProactiveMessage
- backthink.go: think() 用 parseProactive→ShouldPush=push&&disturbAllowed(不再被tr.Import卡死)
- decide.go: disturbAllowed(asleep/working/quiet 硬性不打扰门) 取代旧import门(保留旧Decide兼容)
- 测试: parseProactive 6组+think端到端(真实pusher:kook→IM/无标记→active/no_push→不推/睡眠→门拦截)
   +buildProactiveMessage+具体平台无IM回退; 后端 build/vet/test 全绿
- 注: 生产 Active/IM sink 未注入 wire.go backthink.Deps(另一步); 多平台IM受单IMAdapter接口限制

internal/services/backthink/context.go

@@ -84,6 +84,12 @@ func thinkInput(recent []model.Message, trigger string) []model.Message {
 // never on this prompt text (red line); the prompt only describes the running
 // environment (memories readable/writable, tools callable) that the env already
 // grants.
+//
+// The prompt asks for a compact structured closing — a push/no_push decision, an
+// optional target channel and the message body — so the service can honor her own
+// judgment instead of gating on the trigger's importance (design/20 §2 P2). The
+// format is guidance, not a contract: parseProactive fails safe when she omits or
+// mangles it.
 func guidancePrompt(trigger string) string {
 	return "[自主思考 · 无人在旁]\n" +
 		"触发：" + trigger + "\n" +
@@ -91,5 +97,9 @@ func guidancePrompt(trigger string) string {
 		"请回顾最近与主人的相处、她的状态、你们共同经历的事与临近的日程，判断此刻是否有一件真正值得主动告诉她的、温暖而有针对性的事情——" +
 		"比如关心她的近况、记起她曾说过的某件事、提醒一件即将到来的日程、见证一个属于你们的共同时刻，或注意到某个值得留意的变化。\n" +
 		"若有，就用温暖自然的语气，用一到两句把它说出来——这正是你此刻最想对她说的话；若确实没有值得说的，只需回复「无」，也不要编造。\n" +
+		"请把「是否主动说」与「发到哪个渠道」写成一个简短、格式固定的结尾，解析规则如下：\n" +
+		"[我决定:push] 或 [我决定:no_push]\n" +
+		"[渠道:web|im|kook|telegram|...|auto]（可省略，auto/省略即交给默认：优先网页端、IM 兜底）\n" +
+		"上面两行之后，紧接你想说的话作为正文。\n" +
 		"整轮思考保持简短、单轮，不反复自我触发。"
 }