`store.addNode(null, …)` routes the insert through `_children()` →
`getActivePageChildren(doc, activePageId)` — meaning the parent list
is the ACTIVE PAGE's children, not `doc.children`. The previous
append-index calc read `doc.children?.length` directly, which only
holds the legacy single-page fallback array. On a multi-page doc the
two diverge: `doc.children` may be empty or stale while the active
page already has N siblings, so the computed append index doesn't
correspond to the actual insertion target — landing either before
existing siblings (off-by-N) or out of bounds.
Use `getActivePageChildren(document, activePageId)` to read the same
list `addNode` writes into. Sub-agent generation runs on whichever
page the user has active, so this matches dispatch behavior exactly.
The non-null parent path (`getNodeById(parentId)` then read its
children length) was already correct — only the null-parent branch
needed fixing.
a0e84763d2