Nuum eval sessions pollute session list without hidden root

Nuum eval harness must create all eval sessions as children of a hidden root session — OpenCode hides child sessions from the main list. Without this, eval runs produce ~4000 top-level sessions. The eval root is created once in main() before the concurrency pool starts. Also: backfilled distillations for actively-used sessions get replaced when the live plugin runs distillation cycles — only inactive sessions retain stable backfilled segments.