The pipeline prefetch design was firing \`brv query\` in a background
thread *after* each response, meaning the context injected at turn N
was from turn N-1's message — and the first turn got no BRV context
at all. Replace the async prefetch pipeline with a synchronous query
in \`prefetch()\` so recall runs before the first API call on every
turn. Make \`queue_prefetch()\` a no-op and remove the now-unused
pipeline state.
b3319b1252