Merge fix/2160-opencode-subagent-bootstrap: carry the standalone V1 fix

Both branches implement the same #2160 child-session detection; this
branch's restructured plugin file anchors it differently, so the merge
is resolved by keeping this branch's copy (which also extends the fix
to the V2 context hook). After the standalone PR merges to dev, this
ancestry makes future dev syncs conflict-free.