refactor(hub): role 改为数据条目,不再是写死的闭集枚举

ADR-0017 说 role-based routing 是 product/admin config,不是 spec invariant。
之前 RunRole = "draft"|"review"|"triage" 闭集违反这条——role 钉死在代码里,
教师/admin 无法自定义。改为:
- RoleEntry(id+label+defaultModel)是数据,registry 持有 role 集合
- roleId 退化为 string,新 role 是配置改动不是代码改动
- runner 不再带 role 字段(model 已由 caller resolve 好传入,role 透传无意义)
- 未知 role 优雅降级到首个启用 model,不抛
tsc --noEmit rc=0;role-as-data + ADR-0003 不变式 smoke 通过。

hub/src/server.ts

@@ -24,7 +24,11 @@ function main(): void {
       { id: "z-ai/glm-4.6", label: "GLM-4.6", toolCapable: true },
       { id: "anthropic/claude-3.5-sonnet", label: "Claude 3.5 Sonnet", toolCapable: true },
     ],
-    { draft: "z-ai/glm-4.6", review: "anthropic/claude-3.5-sonnet" },
+    [
+      // Roles are data here — admin/teachers add presets, not code changes.
+      { id: "draft", label: "草稿", defaultModel: "z-ai/glm-4.6" },
+      { id: "review", label: "审校", defaultModel: "anthropic/claude-3.5-sonnet" },
+    ],
   );
 
   const tools = new ToolRegistry();
@@ -36,11 +40,9 @@ function main(): void {
   );
 
   const model = models.resolve(undefined, "draft");
-
   runAgent(provider, tools, {
     prompt: "ping",
     model,
-    role: "draft",
     project: {
       projectId: "proj-skeleton",
       boundChatId: "chat-skeleton",