feat(backthink): 主动推送每次只发一个平台——决策渠道/最近活跃优先，平台含微信

主动推送目标不再取第一个白名单渠道(kook)，也不再双目标 web+kook；改为每次只发一个平台，范围=全部已连接白名单渠道(含微信 weixin-ilink)。

backthink/push.go: pusher 单目标路由——web hint 一次 pushActive；im/auto/实体平台 hint 一次性 pushIM (msgs 置哨兵/实体平台)，无 IM sink 时诚实降级 pushActive；保留 PushGate 降频与成功 RecordPush。
channelbridge/proactive.go: proactiveTarget 三级选择——①决策 [渠道:xxx] 实体平台且白名单+已连接优先；②最近活跃(主会话最近20条 newest-first 找白名单+已连接平台，web 会话自动跳过)；③回退 NextConnectedAccount 用旋转游标避免永远 kook。微信不再排除、可被选中；决策 hint 指定非白名单渠道回退(不报错)。Bridge 新增 proactiveCursor 字段(mu 保护)。
context.go: guidancePrompt 描述改为每次挑一个合适渠道(含微信)。
测试: 决策优先/最近活跃/微信可选中/无信号回退旋转/单目标 等，go build/vet/test 全绿 + windows/darwin 交叉编译。stash@{0} 保留被取代的'web+kook双目标'WIP以备回滚。

internal/services/backthink/push_test.go

@@ -9,14 +9,39 @@ import (
 	"git.yeij.top/AskaEth/Cyrene_v2/internal/channel/session"
 )
 
-func TestPusher_ActiveFirst(t *testing.T) {
+func TestPusher_AutoRoutesIM(t *testing.T) {
 	t.Parallel()
-	active := &mockActive{n: 2}
+	// The owner mandate: a default (auto) push reaches ONE channel over the IM
+	// bridge (the bridge picks her [渠道:...] choice, then her most recent platform,
+	// then the whitelist default) — not the active web client by default.
+	active := &mockActive{n: 2} // a web client is online but is NOT the default target
 	im := &mockIMAdapter{}
 	p := newPusher(active, im, &mockGate{allow: true}, discardLog())
 
 	res := p.Push(context.Background(), PushRequest{Message: adapter.UnifiedMessage{}})
 
+	if !res.Pushed {
+		t.Fatalf("Pushed = false, want true")
+	}
+	if res.Route != RouteIMBridge {
+		t.Fatalf("Route = %q, want %q", res.Route, RouteIMBridge)
+	}
+	if im.count() != 1 {
+		t.Fatalf("im calls = %d, want 1 (single IM target)", im.count())
+	}
+	if active.calls != 0 {
+		t.Fatalf("active calls = %d, want 0 (web is not the default target)", active.calls)
+	}
+}
+
+func TestPusher_WebHintRoutesActive(t *testing.T) {
+	t.Parallel()
+	active := &mockActive{n: 1}
+	im := &mockIMAdapter{}
+	p := newPusher(active, im, &mockGate{allow: true}, discardLog())
+
+	res := p.Push(context.Background(), PushRequest{ChannelHint: channelWeb, Message: adapter.UnifiedMessage{}})
+
 	if !res.Pushed {
 		t.Fatalf("Pushed = false, want true")
 	}
@@ -27,17 +52,19 @@ func TestPusher_ActiveFirst(t *testing.T) {
 		t.Fatalf("active calls = %d, want 1", active.calls)
 	}
 	if im.count() != 0 {
-		t.Fatalf("im calls = %d, want 0 (active should be preferred)", im.count())
+		t.Fatalf("im calls = %d, want 0 (web hint targets only the active client)", im.count())
 	}
 }
 
-func TestPusher_IMFallback_NoActive(t *testing.T) {
+func TestPusher_ConcreteHintRoutesIM_SingleTarget(t *testing.T) {
 	t.Parallel()
-	active := &mockActive{n: 0} // no active client online
+	// A concrete decision channel is honored over the IM bridge, and exactly ONE
+	// target is used (the IM bridge carries it; the active client is not touched).
+	active := &mockActive{n: 2}
 	im := &mockIMAdapter{}
 	p := newPusher(active, im, &mockGate{allow: true}, discardLog())
 
-	res := p.Push(context.Background(), PushRequest{Message: adapter.UnifiedMessage{}})
+	res := p.Push(context.Background(), PushRequest{ChannelHint: "kook", Message: adapter.UnifiedMessage{}})
 
 	if !res.Pushed {
 		t.Fatalf("Pushed = false, want true")
@@ -48,24 +75,31 @@ func TestPusher_IMFallback_NoActive(t *testing.T) {
 	if im.count() != 1 {
 		t.Fatalf("im calls = %d, want 1", im.count())
 	}
+	if active.calls != 0 {
+		t.Fatalf("active calls = %d, want 0 (a concrete hint targets only the IM bridge)", active.calls)
+	}
+	if im.last.Platform != "kook" {
+		t.Fatalf("im platform = %q, want kook", im.last.Platform)
+	}
 }
 
-func TestPusher_IMFallback_ActiveError(t *testing.T) {
+func TestPusher_NoIMSink_DegradesToActive(t *testing.T) {
 	t.Parallel()
-	active := &mockActive{err: context.DeadlineExceeded}
-	im := &mockIMAdapter{}
-	p := newPusher(active, im, &mockGate{allow: true}, discardLog())
+	// With no IM sink wired (backthink IM seam absent) the push degrades to the
+	// active web client so the owner is still reached rather than dropped.
+	active := &mockActive{n: 1}
+	p := newPusher(active, nil, &mockGate{allow: true}, discardLog())
 
 	res := p.Push(context.Background(), PushRequest{Message: adapter.UnifiedMessage{}})
 
 	if !res.Pushed {
 		t.Fatalf("Pushed = false, want true")
 	}
-	if res.Route != RouteIMBridge {
-		t.Fatalf("Route = %q, want %q", res.Route, RouteIMBridge)
+	if res.Route != RouteActiveClient {
+		t.Fatalf("Route = %q, want %q", res.Route, RouteActiveClient)
 	}
-	if im.count() != 1 {
-		t.Fatalf("im calls = %d, want 1", im.count())
+	if active.calls != 1 {
+		t.Fatalf("active calls = %d, want 1", active.calls)
 	}
 }