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.

packages/core/agent/README.md

@@ -32,11 +32,9 @@ The full `agent/*` event taxonomy is declared via declaration merging in `dsh-ag
 - `agent/status` — idle / running / disposed transition
 - `agent/queued` — message entered inbox (source-resolved, steering flag)
 
-#### Turn boundaries (emit)
+#### Boundaries are durable session events, not `agent/*` emits
 
-- `agent/turn-start`, `agent/turn-end` (carries `TurnEndReason`)
-
-Step boundaries are NOT mirrored as `agent/*` emits: a consumer that needs per-step boundaries reads the durable `step/start`/`step/end` session events (the session log is the live boundary feed). The turn boundaries stay as `agent/*` emits because the stdio UI needs the `Agent` handle (`agent.id`) at the boundary, which the session event does not carry. See [the event-domain-semantics RFC](../../../docs/rfc/implemented/architecture/2026-06-30-event-domain-semantics.md).
+Turn and step boundaries are NOT mirrored as `agent/*` emits: a consumer that needs them reads the durable `turn/start`/`turn/end`/`step/start`/`step/end` events off the `session/event` feed (the session log is the live boundary feed, carrying the `Session` — the turn/step numbers and reasons ride on the event data). See [the event-domain-semantics RFC](../../../docs/rfc/implemented/architecture/2026-06-30-event-domain-semantics.md) and [the remove-boundary-mirror-events RFC](../../../docs/rfc/implemented/simplification/2026-06-20-remove-agent-boundary-mirror-events.md).
 
 #### Interception seams
 

packages/core/agent/src/types.ts

@@ -26,13 +26,12 @@
  *
  * **The rule:** a durable, replayable fact is a SessionEvent; a live
  * interception or a transient/live-object signal is an `agent`/`tools` Cordis
- * event. A datum that is BOTH (a turn/step boundary) lives in the session log,
- * and is mirrored as an `agent/*` emit ONLY where a live consumer provably
- * needs the `Agent` handle at that instant. Turn boundaries are so mirrored
- * (the stdio UI labels output by `agent.id`); step boundaries are NOT (no live
- * consumer needs them — read `step/start`/`step/end` from the session log).
+ * event. A turn/step boundary is a durable fact: it lives in the session log
+ * and is read off the `session/event` feed — it is NOT mirrored as an `agent/*`
+ * emit. A consumer that needs the `Agent` handle (or its short id) at a boundary
+ * keeps a session-id→agent map from `agent/created`/`agent/disposed`.
  * See `docs/rfc/implemented/architecture/2026-06-11-microkernel-event-taxonomy.md`
- * and the related `docs/rfc/proposed/simplification/2026-06-20-remove-agent-boundary-mirror-events.md`.
+ * and `docs/rfc/implemented/simplification/2026-06-20-remove-agent-boundary-mirror-events.md`.
  *
  * @module @deepseek-ai/dsh-agent/types
  */
@@ -47,7 +46,7 @@ export type AgentId = Branded<'AgentId'>
 export function AgentId(id: string): AgentId {
   return id as AgentId
 }
-import type { Session, TurnEndReason } from '@deepseek-ai/dsh-session'
+import type { Session } from '@deepseek-ai/dsh-session'
 
 /**
  * Options an agent is created with.
@@ -183,26 +182,11 @@ declare module 'cordis' {
      */
     'agent/queued'(agent: Agent, content: ContentBlock[], info: { source: MessageSource; steering: boolean }): void
 
-    // ---- turn boundaries (emit) — the live boundary surface ----
-    // Step boundaries are NOT mirrored here: a consumer that needs per-step
-    // boundaries reads the durable `step/start`/`step/end` session events (the
-    // session log is the live transcript feed). The TURN boundaries stay as
-    // agent/* emits because the only live consumer (the stdio UI) needs the
-    // `Agent` handle at the boundary to label output, which the session event
-    // does not carry. See the module doc's three-domain rule.
-    /**
-     * A turn began. `turn` is the 1-based turn number within the session.
-     * @mode emit
-     */
-    'agent/turn-start'(agent: Agent, turn: number): void
-    /**
-     * A turn ended. `reason` distinguishes a clean stop from a truncated,
-     * aborted, failed, disposed, or crash-interrupted one (`completed` |
-     * `aborted` | `error` | `disposed` | `max-tokens` | `interrupted`); the
-     * reason union is merge-extensible, so a plugin can add further variants.
-     * @mode emit
-     */
-    'agent/turn-end'(agent: Agent, turn: number, reason: TurnEndReason): void
+    // Turn and step boundaries are NOT mirrored as agent/* emits: a consumer
+    // that needs them reads the durable `turn/start`/`turn/end`/`step/start`/
+    // `step/end` session events off the `session/event` feed (the session log is
+    // the live transcript feed). See the module doc's three-domain rule and the
+    // "remove agent boundary mirror events" RFC.
 
     // ---- step/request extension seams (serial + waterfall) ----
     /**