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/background-task/tools.ts

@@ -9,3 +9,4 @@ export type {
 export { createBackgroundTask } from "./create-background-task"
 export { createBackgroundOutput } from "./create-background-output"
 export { createBackgroundCancel } from "./create-background-cancel"
+export { createBackgroundWait } from "./create-background-wait"