refactor(events): remove the turn boundary mirror events

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.

docs/rfc/implemented/simplification/2026-06-20-remove-agent-boundary-mirror-events.md

@@ -0,0 +1,37 @@
+# RFC: Stop mirroring durable boundaries as agent events
+
+Status: implemented (accepted 2026-07-01)
+
+<!-- Shipped in AMENDED, narrowed form: the four turn/step BOUNDARY mirrors are
+     removed; `agent/steering` and `agent/stream-chunk` are RETAINED (they are
+     not durable-boundary mirrors — see "Scope: what is and isn't removed"). The
+     original proposal bundled `agent/steering` into the removal; validating
+     against the code showed it is a distinct live-only signal, so it stayed. -->
+
+## Problem
+
+The loop records the canonical transcript in `SessionEvent` and also emitted a parallel set of live `agent/*` boundary mirror events: `agent/turn-start`, `agent/turn-end`, `agent/step-start`, and `agent/step-end`. The mirrors made consumers choose between two sources of truth for the SAME durable fact. ACP already chose the session log for the editor-facing transcript because a throwing peer listener can prevent later `agent/*` listeners from observing a boundary, while the session event was already appended. The stdio UI was the only production consumer that still rendered turn boundaries from the mirror events; it already rendered tool calls and results from `session/event`.
+
+This duplication is not free. Every lifecycle change had to update the session event, the mirror event, docs, invariants, tests, and snapshot expectations. The duplicate boundary events also made failure ordering subtle: a turn can be durably closed before a live `agent/turn-end` listener runs, so a post-boundary listener failure has no valid in-log position left and must be reported out of band.
+
+## Decision
+
+Make `session/event` the single live boundary/transcript stream. Consumers that render turns, tool calls, tool results, assistant messages, and durable boundaries subscribe to `session/event` and derive their UI from the same event vocabulary persistence uses.
+
+The four durable-boundary mirrors — `agent/turn-start`, `agent/turn-end`, `agent/step-start`, `agent/step-end` — are removed from the agent event taxonomy. A UI that wants the agent handle (or its short id) at a boundary keeps a small map from session id to agent id built from `agent/created`/`agent/disposed`; `dsh-ui-stdio` does exactly this to label its `[<agent> turn N]` header, since the `turn/start` session event carries only the turn number. The canonical record remains the event-sourced session log.
+
+The step mirrors (which had no consumer at all) were removed first, in [the event-domain-semantics RFC](../architecture/2026-06-30-event-domain-semantics.md); that RFC KEPT the turn mirrors on the stated justification that the stdio UI needed the `Agent` handle at the turn boundary. This RFC finishes the job: `dsh-ui-stdio` is a disposable test REPL whose rendering can change freely, so "ui-stdio needs it" is not a reason to keep a mirror — it was migrated to `session/event` + the id map, and the turn mirrors were removed too.
+
+## Scope: what is and isn't removed
+
+Removed (durable-boundary mirrors — the session log is authoritative for each): `agent/turn-start`, `agent/turn-end`, `agent/step-start`, `agent/step-end`.
+
+RETAINED — NOT durable-boundary mirrors, so out of scope for this decision:
+
+- `agent/steering` — a live control signal, not a boundary. (The original proposal bundled it into the removal; validating against the code, it is not a duplicate of a durable boundary, so removing it here would have been scope creep. Its fate is a separate future decision.)
+- `agent/stream-chunk` — the live token stream. `assistant/chunk` persistence remains load-bearing, so the chunk stream could later be evaluated as a mirror, but that is a separate decision.
+- `agent/created`, `agent/disposed`, `agent/status`, `agent/error`, `agent/queued` — lifecycle/control events that are not transcript data. `agent/queued` in particular is an inbox acknowledgement that fires before any durable event exists (cancelled queued work may never enter the log), so it is deliberately live-only.
+
+## What we give up
+
+A plugin can no longer observe turn/step boundaries from a convenient `Agent`-first event. It must either subscribe to `session/event` or maintain a session-to-agent association. That is an acceptable trade: boundary consumers should not depend on a second event feed that can drift from the durable log.