merge(dev): resolve background-agent delegated fallback conflicts

Reconcile the latest dev branch changes with the delegated child-session fallback work. Preserve the upstream background-agent updates while keeping the delegated bootstrap cleanup and compatibility wiring fixes intact, then re-verify the affected regression suites and typecheck.

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

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

src/tools/delegate-task/background-task.ts

@@ -190,7 +190,6 @@ export async function executeBackgroundTask(
     const taskMetadataBlock = sessionId
       ? `\n\n${buildTaskMetadataBlock({
         sessionId,
-        taskId: sessionId,
         backgroundTaskId: task.id,
         agent: task.agent,
         category: args.category,