service: turn replies are not streamed

[edwin-zvs]

A service turn's reply is returned whole. POST /svc/<name> runs the turn and answers with the final text; there is no incremental delivery, so a caller sees nothing until the harness is completely done.

For an agent turn that can run for minutes this is a poor fit, and it is the reason the approval-reporting work (#1113 / spec 0171) had to exist in the shape it does: with no stream, a caller cannot otherwise distinguish "blocked on a human" from "still working", so that state had to be exposed through the result-polling path instead.

Design questions:

• Transport. SSE is the obvious fit for the HTTP channel and needs no client change beyond reading a stream. It does not generalize to Slack, which wants periodic message edits instead — so this is likely per-channel behavior over a shared incremental-event source rather than one mechanism.
• Granularity. Token deltas, or coarser milestones (turn started, tool call, reply)? Note the reply-truncation bug (fix(service): reassemble streamed assistant replies #1107) came from smith emitting one message event per token delta — the daemon already sees fine-grained events and currently reassembles them; streaming means choosing what to forward rather than building new plumbing.
• Interaction with parked approvals. A stream is the natural place to surface "waiting on approval" as it happens rather than on poll.