Complete the boundary-mirror removal begun with the step mirrors: drop
`agent/turn-start` and `agent/turn-end` from the agent event taxonomy. Turn and
step boundaries are now read exclusively off the durable `session/event` feed
(`turn/start`/`turn/end`/`step/start`/`step/end`) — there is no `agent/*` mirror
for any boundary.
- loop.ts: delete both turn emits; `closeTurn` loses its `emit` parameter and
its now-unreachable idempotency guard (it is called exactly once per turn, on
mutually exclusive normal/catch paths); `failTurn` loses the dead post-close
branch that only a throwing turn-end LISTENER could reach.
- ui-stdio: render turn boundaries from `session/event`, recovering the short
agent label from an `agent/created`→id map (the `turn/start` event carries only
the turn number, and the session id is not reliably the agent id). ui-stdio is
a disposable test REPL, so this migration retires the sole justification the
event-domain-semantics RFC gave for KEEPING the turn mirrors.
- Tests: reason/turn-number collectors and the boundary-ordering test now read
`session/event`; the throwing-turn-boundary-LISTENER tests are deleted (that
code path no longer exists). A new test covers the outer-catch disposed branch
via a pre-step listener that disposes-then-throws (the surviving real path).
- Docs: promote the "remove agent boundary mirror events" RFC to implemented
(amended/narrowed — `agent/steering` is RETAINED, not a boundary mirror);
update the event-domain-semantics + turn-enclosure RFCs, architecture.md, the
cookbook, the ACP/agent/ui-stdio prose, and regenerate the cordis catalog.
`agent/steering` and `agent/stream-chunk` are explicitly out of scope (not
durable-boundary mirrors). ACP is unaffected — it already settles from the log's
`turn/end` + `agent/status`; snapshot goldens are byte-unchanged.
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