Codex review caught an observe-only violation: the subagent/end emit fires from a
detached `.then` registered BEFORE start() returns — so before the caller's own
`await run.result` continuation runs. Carrying `result.output` by reference let a
mutating subagent/end listener corrupt the SubagentResult.output the caller/tool
then consumes. structuredClone() makes the event a read-only snapshot. Added a
regression test that mutates the event's array and asserts the caller's result is
untouched; proven to fail red without the clone. Updated the RFC + READMEs to note
the clone is load-bearing for the observe-only guarantee.
Tianyi Cui
93106b87b4