Agent-list expansion (compact "N agents" summary + per-parent lineage folds) lived in local useState on WorktreeCardAgents, so the WorktreeCard remount triggered by the child-worktrees toggle (a virtual-row key flip between item and lineage-group) and by virtualizer scroll-recycle wiped it — folding one section reset the other. Lift it into a session-scoped, LRU-bounded per-worktree module cache (useWorktreeAgentExpansionState), mirroring pr-comments-list-selection. Adds unit + integration + e2e regression tests. Reviewed for regressions and performance.
