## Summary
Long-running chat agents were filling `session.out` forever — every
`chat.agent` turn appended to the same S2 stream with no trim, and the
Sessions dashboard re-streamed the entire history from `seq_num=0` on
every page load. After this change the agent appends an S2 `trim`
command record after each `trigger:turn-complete`, pointing back at the
previous turn-complete's seq_num. `session.out` stays roughly one turn
long at steady state, regardless of session age.
`trigger:turn-complete` and `trigger:upgrade-required` move from
`chunk.type`-shaped data records into header-form control records under
a uniform `trigger-control` namespace. Built-in transports
(`TriggerChatTransport`, `AgentChat`, the dashboard's `AgentView`)
handle the new shape transparently. Custom transports need a one-line
filter on the `trigger-control` header — see the rewritten "Records on
session.out" section in the client-protocol docs.
The Sessions detail page in the dashboard fetches the agent's per-turn
S3 snapshot via a presigned URL and seeds the transcript view, then
SSE-tails from the snapshot's `lastOutEventId`. Bandwidth and
time-to-first-render scale with unread turns instead of session
lifetime.
Resume contract is now explicit: single-turn-boundary resume always
works (the prior turn-complete is still on the stream), the S2 trim is
eventually consistent over 10-60s, and multi-turn-away resume falls back
to a snapshot reload.
Eric Allam
82853debea