Accept tags as an array in item_store and batch #369

Problem

In a batch store, tags must be a comma-separated string; passing a JSON array fails the whole batch. Agents naturally send arrays.

Proposal

Accept both a string and an array of strings for tags (single and batch), normalising server-side; the error message should name the field and the accepted shapes.

Evidence

Hyperion Drift 2026-09-26: a roadmap batch failed on tags as array and had to be resent.