feat(athena): add background_wait tool for race-style task collection

New tool that takes multiple task IDs and blocks until ANY one completes (Promise.race pattern). Returns the completed task's result plus a progress summary with remaining IDs. Enables Athena to show incremental council progress without polling.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/tools/index.ts

@@ -25,6 +25,7 @@ export { createSkillMcpTool } from "./skill-mcp"
 import {
   createBackgroundOutput,
   createBackgroundCancel,
+  createBackgroundWait,
   type BackgroundOutputManager,
   type BackgroundCancelClient,
 } from "./background-task"
@@ -53,6 +54,7 @@ export function createBackgroundTools(manager: BackgroundManager, client: Openco
   return {
     background_output: createBackgroundOutput(outputManager, client),
     background_cancel: createBackgroundCancel(manager, cancelClient),
+    background_wait: createBackgroundWait(outputManager, client),
   }
 }