learningsCopyFromProject called learningsWrite K times in one process,
and each call re-scanned the entire learnings store to dedupe — O(K*N).
Build the content_hash -> id index once at the start of the bulk import
and thread it through; single-write behavior and the return contract are
unchanged (no caller reads `id` on the created:false branch). O(K*N) ->
O(N+K). Adds a regression test asserting store scan count is independent
of import size.
The larger persistent on-disk index (atomic updates, corruption rebuild,
cross-process dedupe) is deferred — needs design decisions and is not
required to resolve the bulk-import scan this issue reports.
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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