Introduces b/ as a first-class 9P namespace alongside s/. Writing to
b/new creates one or more ephemeral agent jobs (parallel=N suffix -0..-N)
each running the same prompt in an independent agentCore with no
persistent session state.
Per-job files: spec, status, result, usage, ctxsz.
BatchStore/BatchJobStore mirror SessionStore/SessionFileStore.
rm -r b/{id} cancels and removes the job.
Also removes s/{id}/reply: b/{id}/result is the purpose-built
replacement for one-shot prompt->reply use cases.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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