feat(backthink): 自主思考强化 P1——主会话上下文+owner env+去压制(design/20)

backthink 从"心跳空转(独立sess-backthink+写死nothing to report宏)"改成"看得见主人、
能读写记忆、能调工具"：think 读主会话最近20条(history.Recent,倒序映射)作为上下文 +
 重写引导prompt(温暖/有针对性/主动表达,删掉 Do not produce chat-style message) + 注入
 loop.OwnerEnv(MemWritable=true/ActorOwner/全工具)。
- context.go: mainContext/recentProvider/thinkInput/guidancePrompt
- thinker.go: LoopThinker 加 recent;Think 读主会话上下文+OwnerEnv+MutedBroadcast
- wire.go: NewLoopThinker(a.loop, a.history)
- 刻意偏离(安全): loop.Run session id 仍用 sess-backthink(不写"web"),避免与用户实时回合
  checkpoint/中断窗口复用污染;上下文来源+运行环境=主会话/owner env,目标达成。红线靠actor元数据。
- 测试: prompt无压制/有鼓励;主会话消息映射;nil退化;端到端Think(owner env 生效)
验收: build/vet/test 全绿+windows/darwin交叉编译绿. P2(主动推送+渠道选择)后置.

internal/services/backthink/backthink.go

@@ -55,8 +55,12 @@ const (
 	// defaultThinkTimeout bounds a single unattended thought so it can never
 	// wedge a worker goroutine.
 	defaultThinkTimeout = 30 * time.Second
-	// defaultSessionID scopes a think when the caller supplies none. The
-	// background pass owns no conversation, so a stable synthetic id is used.
+	// defaultSessionID scopes a think when the caller supplies none. It is the
+	// loop.Run durability key, deliberately distinct from history.MainSessionID so
+	// an autonomous think never collides with the live main session's checkpoint
+	// store (channelbridge saves/resumes under the main id). The think still sees
+	// the main conversation: LoopThinker reads history.MainSessionID for context and
+	// runs under loop.OwnerEnv (design/20 §2).
 	defaultSessionID = "sess-backthink"
 )