docs(agents): clarify background delegation durability

[bbopen]

Summary

Clarifies the delegate_task guidance in AGENTS.md so it matches the behavior Hermes already has today:

• default delegate_task waits for the child summary before the parent continues
• background=true returns a delegation id immediately and completes later through the async-delegation completion queue
• background delegation is detached from the current turn but remains process-local, so restart-durable work should still use cronjob or terminal(background=True, notify_on_complete=True)

Validation

• Reviewed the existing tools/delegate_tool.py background path that removes the child from the parent turn and hands it to async delegation.
• No automated tests run; this is an AGENTS.md documentation-only change.

[teknium1]

Merged via PR #49944 (docs salvage cluster 2). Your commit was cherry-picked onto current main with your authorship preserved in git log — rebase-merged so it lands under your name. Thanks for the contribution!