list agents to be used as subagents (#278624)

* instructions list: use XML and tool variables

* update

* update

* Merge branch 'main' into aeschli/xenial-felidae-488

* allow to run agents as subagents (#278971)

* fix

* Merge remote-tracking branch 'origin/main' into aeschli/xenial-felidae-488

* fix

* fix