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/mocks_test.go

@@ -52,16 +52,18 @@ func (m *mockState) Current(context.Context) (UserState, error) { return m.state
 
 // mockPusher records Push calls and returns a fixed result.
 type mockPusher struct {
-	mu      sync.Mutex
-	result  PushResult
-	calls   int
-	lastMsg adapter.UnifiedMessage
+	mu       sync.Mutex
+	result   PushResult
+	calls    int
+	lastMsg  adapter.UnifiedMessage
+	lastHint string
 }
 
-func (p *mockPusher) Push(ctx context.Context, msg adapter.UnifiedMessage) PushResult {
+func (p *mockPusher) Push(ctx context.Context, req PushRequest) PushResult {
 	p.mu.Lock()
 	p.calls++
-	p.lastMsg = msg
+	p.lastMsg = req.Message
+	p.lastHint = req.ChannelHint
 	p.mu.Unlock()
 	return p.result
 }