Closes#78. The 30s server WriteTimeout is an absolute deadline that was cutting
every agent turn over 30s mid-stream — and the keep-alive from #73 could never
work because it ticked into a connection destroyed at 30s. Refresh a per-write
deadline instead: unbounded stream, bounded writes, so a stuck reader still can't
pin the run goroutine. Two client-side regression tests, one per failure mode.
Gadfly: no material issues on final review.
steve