The share-code codec encodes createdBy through a fixed table that only
knew none/agent/user, so idxOf returned 0 for 'learn' and a learned
skill round-tripped as createdBy: null. The 2-bit field has a free slot,
so 'learn' now occupies it, and metaBadges labels agent- and
learn-created skills alike as 'learned' — matching the Python side,
where both values count as learning signals.
Review finding: learn nodes lost createdBy in share-code round-trip.
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