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/options.go

@@ -92,6 +92,13 @@ type Options struct {
 	// scopes memory retrieval and extraction, the system prompt fallback and the
 	// emotion hook. Blank skips memory injection/extraction.
 	PersonaID string
+	// PersonaName, when non-nil, resolves a role id to its display name for the
+	// context's speaker attribution (design/22 §7-5): the role declaration and the
+	// boundary notes name a speaking role as "「昔涟」（xilian）" instead of a bare id,
+	// so the model can follow "who said this" without a lookup table of its own. An
+	// unknown role, an empty name or a nil resolver all fall back to the id — the
+	// attribution never disappears because a name could not be resolved.
+	PersonaName func(personaID string) string
 	// PersonaKnobs, when non-nil, resolves the reasoning knobs for a role at the
 	// start of every request build (design/22 §4.2), so a session bound to role Y
 	// uses Y's thinking budget / effort / broadcast switch rather than the default