bug(sessions): reloading a live child while its parent is closed drops the child's parent pointer for good #1379

Problem

If a LIVE child is reloaded or replaced while its parent pane is CLOSED (on the undo stack), the child loses its parent pointer for good. remapSessionsRelationships (session.ts replace and Reload Agents commits) drops any linkedParentId / orchestrationParentId / orchestrationRootId whose target is in neither the idMap nor the record's own session ids, and a closed parent is in neither. When the parent is later restored with Undo Close, #1374 remaps only pointers that still name it, and this child's pointer is already gone. Both panes are live but permanently unlinked: the child renders top-level, and the restored parent cannot see, prompt or close it through orchestration.

Evidence

• Found by review c of fix(undo-close): a restored parent's live children follow it to its new id #1374 (temp/review-1374/report-c.md, "Reverse-direction pre-existing gap").
• fix(undo-close): a restored parent's live children follow it to its new id #1374 deliberately KEEPS unmapped pointers on the undo path (remapMetaLineage) for exactly this reason. The replace/reload path's drop is the mirror image.

Likely fix

On the replace/reload commits, keep a pointer whose target is still on the undo stack, rather than dropping everything that is not live. That needs the undo stack's closed ids at the commit.

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