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/runtime/loop/sessionenv.go

@@ -61,6 +61,15 @@ func WithSessionEnv(ctx context.Context, env SessionEnv) context.Context {
 	return context.WithValue(ctx, sessionEnvKey{}, env)
 }
 
+// SessionEnvFrom returns the environment attached to ctx by WithSessionEnv, or the
+// owner-full default when none was attached (backward compatible). It is the read
+// side of WithSessionEnv, for a caller that receives only a context — a Chatter
+// implementation that must know the turn's role/isolation boundary, or a test
+// asserting what an inbound path resolved. The value is display/scope metadata
+// only: identity and permission were already decided by the actor metadata before
+// the env existed (project red line).
+func SessionEnvFrom(ctx context.Context) SessionEnv { return sessionEnvFrom(ctx) }
+
 // sessionEnvFrom returns the env attached to ctx, or the owner-full default when
 // none was attached (backward compatible).
 func sessionEnvFrom(ctx context.Context) SessionEnv {