Rebuild a collection's vector index

RE-EMBEDS EVERY FILE IN THE COLLECTION. This costs money.

Embedding is charged per token, so the cost is proportional to the whole collection, not to whatever changed. Use it to recover from a bad index, not as a routine refresh - adding a file already indexes that file.

Tiered billable by an explicit rule rather than by name: nothing in the word "reindex" matches a billable path marker, and as a plain write this would ride along with "rename a collection".