refactor: add execution backend selection

docs/superpowers/plans/2026-04-02-boss-execution-foundation.md

@@ -610,8 +610,9 @@ export async function selectExecutionBackend(input: {
   primary: { provider: string; status: string };
   backups: Array<{ provider: string; status: string }>;
 }) {
-  if (input.primary.provider === "master_codex_node" && input.primary.status === "ready") {
-    return { backendId: "master-codex-node" };
+  const primaryBackend = resolveBackendByProvider(input.primary.provider);
+  if (input.primary.status === "ready") {
+    return primaryBackend;
   }
   const qwen = input.backups.find((item) => item.provider === "aliyun_qwen_api" && item.status === "ready");
   if (qwen) {
@@ -621,12 +622,20 @@ export async function selectExecutionBackend(input: {
   if (openai) {
     return { backendId: "openai-api" };
   }
-  return { backendId: "master-codex-node" };
+  const master = input.backups.find((item) => item.provider === "master_codex_node" && item.status === "ready");
+  if (master) {
+    return { backendId: "master-codex-node" };
+  }
+  return primaryBackend;
 }
 
 export const selectExecutionBackendForTesting = selectExecutionBackend;
 ```
 
+补充说明：
+- selector 的运行时语义以“`ready primary` 优先，否则按 `aliyun_qwen -> openai -> master_codex_node` 顺序回退，最后再回 primary 兜底”为准。
+- 如果同一 provider 存在多个账号，只要其中任何一个是 `ready`，该 backend 就视为可选。
+
 在 `/Users/kris/code/boss/src/lib/boss-master-agent.ts` 中，把 provider fallback 的选择逻辑收进一个单独调用：
 
 ```ts