Review #3 (LOW). `divergence_preview` (plan path) compared parent slugs
case-sensitively from its own `get_by_slug`, while the apply path
(`reconcile_group_structure_tx`) compares resolved parent ids. On a mixed-case
parent slug the two could disagree — `pic plan` reporting `diverged` while
`pic apply` hard-errors "parent does not exist".
`resolve_existing_group_ids` becomes `load_existing_groups`, returning the full
`Group` rows it already loads; `plan_tree` derives the id map from them and
passes the map to `divergence_preview`, which now reads each node's own row
without a per-node `get_by_slug` (kills the N+1), resolves an in-tree parent's
slug from the same map, and compares parent slugs case-insensitively so the
preview agrees with the apply path's id-based check.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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