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/channelbridge.go

@@ -140,6 +140,14 @@ type Options struct {
 	// (channels.json → env fallback). When non-nil the bridge supports hot reload;
 	// nil keeps the bridge boot-only (Reload returns ErrNilFactory).
 	Factory PlatformFactory
+	// ChannelPersona, when non-nil, resolves the role bound to one channel account
+	// (channels.json ChannelAccount.PersonaID, design/22 §2 方案 b): it is called with
+	// the platform and the local account id, and returns "" when the account binds no
+	// role. It is consulted once per adapter connect/reload (never per message) and
+	// only for the owner's private chat, so a group/stranger conversation can never be
+	// moved into a role's scope (design/22 §7-4). Nil keeps every channel on the global
+	// default role.
+	ChannelPersona func(platform, accountID string) string
 }
 
 // Bridge owns the channel adapter lifecycle and the inbound message pipeline. It
@@ -181,6 +189,12 @@ type Bridge struct {
 	started     bool
 	closed      bool
 	factory     PlatformFactory
+	// channelPersona resolves an account's channel-level role from the effective
+	// config (design/22 §2 方案 b); adapterPersona caches the resolved role per
+	// connected adapter so the inbound path never re-reads channels.json. Both are
+	// guarded by mu.
+	channelPersona func(platform, accountID string) string
+	adapterPersona map[string]string
 	// proactiveCursor is the rotation cursor for the proactive-push no-signal
 	// fallback (channelbridge/proactive.go), so the default channel is not always
 	// the first whitelisted owner channel. Guarded by mu.
@@ -251,6 +265,8 @@ func New(opts Options) (*Bridge, error) {
 		readCancels:    make(map[string]context.CancelFunc),
 		gates:          make(map[string]*adapter.FlowGate),
 		factory:        opts.Factory,
+		channelPersona: opts.ChannelPersona,
+		adapterPersona: make(map[string]string),
 		cancel:         func() {},
 		sem:            make(chan struct{}, maxConcurrent),
 	}, nil