fix(muse): list background children under their turn item's native id

Muse listed running children on the background roster under itemId while their subagent turn items use childSessionId, then subagentId, then itemId. The roster now uses the same id and follows it as Muse reveals the child's ids.

Fixes #23844
8cf8dba435Koushik committed on 10/22/2026, 9:25:45 PMparent2c8bdf6

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