feat(orchestration,runtime): subagent_spawn 支持指定目标角色（以该角色 instructions + 记忆域运行）

design/22 §8-2（主人 2026-10-03 拍板）：跨角色协作 = 把其他角色以子代理形式派出去，
该子代理以目标角色的 instructions 运行、并按目标角色的记忆作用域读写。

- AgentSpec / SubAgent / SubAgentResult 新增 PersonaID；Options.PersonaExists 作角色解析器
- scheduler：派发前校验角色（ErrUnknownPersona），批量 all-or-nothing；
  run ctx 经 withPersonaEnv 只覆盖 SessionEnv.PersonaID —— ActorEnv/MemWritable 原样复制
  （身份识别靠 channel 元数据，persona 决定她怎么答、不决定她能碰什么：项目红线）
- 记忆归属键改为 PersonaID 优先（回落 RoleID）→ 替班者写的是目标角色的记忆域
- spawntool：schema 顶层与 tasks[] 新增可选 persona；省略 = 与旧行为逐字节一致
- 分支根节点新增 props.persona（可观测性）；SinkMemory 移入 memory.go（守 500 行红线）
- 组合根：personaExists(agent 池)；cmd/cyrene/wire.go +5 行

契约：Docs/api-client.md -> v0.47（graph_update 新增可选 props.persona，纯增量）

验证：go build/vet ./... 绿；internal/runtime/orchestration + cmd/cyrene 测试绿；
      go test -race ./internal/runtime/orchestration -run Persona 绿；
      linux/amd64 + darwin/arm64 交叉编译绿
变异验证：① 目标角色换回父/默认角色（3 处断言 FAIL）② 关调度器侧角色校验 ③ 关工具侧预检

internal/runtime/orchestration/options.go

@@ -68,6 +68,15 @@ type Options struct {
 	// Approve. Policy judgement belongs to the upper layer. Optional.
 	Approver *audit.Approver
 
+	// PersonaExists reports whether a role id can run with its own instructions,
+	// i.e. whether the per-role agent pool behind Loop (loop.Options.Agents) has an
+	// entry for it (design/22 §8-2). It gates AgentSpec.PersonaID: a dispatch naming
+	// an id this returns false for is rejected with ErrUnknownPersona rather than
+	// running as the default role's prompt. Nil means "this deployment knows no
+	// role", so a dispatch that names a role is rejected too — deciding nothing
+	// silently is the point. Omitting the persona is unaffected by this.
+	PersonaExists func(personaID string) bool
+
 	// Logger is the base logger. Nil is replaced with a discard logger tagged with
 	// source "orchestration".
 	Logger *slog.Logger