feat(channel,runtime): 批次2 后端 — 消息级角色归属进上下文 + IM 私聊随主会话角色 + 渠道级绑定

按 design/22 §2/§3/§4.4/§7（3/4/5）。
- ①「消息级角色归属」进入模型上下文（批次 1 只做存储+DTO）：历史消息的发送角色现在对模型可见，且**不把系统元数据塞进用户正文**（design/11 §10 红线）
- ② IM owner 私聊镜像解析出的角色 = **主会话绑定的角色**（§7-3）
- ③ 渠道级绑定：ChannelAccount 加 personaId（channels.json 读写/校验/脱敏沿用既有约定）+ 角色解析优先级 **会话 > 渠道 > 默认** + **群聊/陌生人强制默认角色**（§7-4）；admin 渠道配置端点支持 personaId
- Docs/api-client 版本 +1
- 验收：go build/vet/test 全绿 + linux/darwin 交叉编译

internal/services/channelbridge/reload.go

@@ -74,6 +74,10 @@ func (b *Bridge) connectAndRun(a adapter.PlatformAdapter) {
 	b.readCancels[a.Name()] = rcancel
 	b.connected[a.Name()] = true
 	b.mu.Unlock()
+	// Cache the account's channel-level role now that the adapter is live: the
+	// inbound path reads the cache instead of the config, and a reload (which the
+	// panel's save triggers) re-runs this to pick up a new binding.
+	b.rememberAdapterPersona(a.Name())
 	b.wg.Add(1)
 	go b.readLoop(a, readCtx)
 	b.log.Info("channel connected", "channel", a.Name())
@@ -169,5 +173,6 @@ func (b *Bridge) removeAdapter(o adapter.PlatformAdapter, c context.CancelFunc)
 	delete(b.connected, name)
 	delete(b.readCancels, name)
 	b.mu.Unlock()
+	b.forgetAdapterPersona(name)
 	b.log.Info("channel (reload) removed", "channel", name)
 }