Clarify subagent coordination cards

SUBAGENTS.md

@@ -33,6 +33,13 @@ does not repeat the completed item's `delivery` field, so the translator
 remembers that field from `item/started` and suppresses those deltas. Letting
 one into the recipient's provisional assistant row makes its next completed
 message replace the combined row, visibly erasing text that Codex still has.
+The parent draws the initial `spawnAgent` as its ordinary `Task` card and
+closes it when the matching `subAgentActivity.started` arrives. The remaining
+collaboration calls remain visible as coordination -- waiting, messaging,
+listing and lifecycle controls -- rather than being mistaken for generic task
+output. Null optional fields and a bare `completed` status carry no information
+and are omitted; their useful result is the child transcript, status or peer
+message beside them.
 
 ## Storage