docs(events): fix stale turn-mirror references caught in review

Codex review of the turn-mirror removal found current-state docs/comments that
still claimed the removed `agent/turn-start`/`agent/turn-end` events exist:

- docs/architecture.md: the loop diagram's turn-start line still said "emit
  agent/turn-start" (the turn-end line was already fixed).
- event-domain-semantics RFC: the `agent/*` domain description listed "the turn
  boundaries" among the transient emits.
- docs/core-data-structures/core.md: the agent/* taxonomy blurb listed
  "turn/step boundaries" as agent events.
- the proposed ACP RFC: the settle-signal rows named agent/turn-start /
  agent/turn-end; retargeted to the durable `turn/end` session event + the
  session/event owning-turn correlation.
- loop.ts outer-catch comment: said "closeTurn/failTurn are idempotent" — after
  the emit-param removal closeTurn is called exactly once (mutually exclusive
  normal/catch paths), so corrected to state that and to scope idempotency to
  closeStep (which is still guarded by stepOpen).

Regenerated the cordis catalog. No behavior change.

docs/core-data-structures/core.md

@@ -306,7 +306,7 @@ interface Agent {
 }
 ```
 
-`AgentStatus` is `'idle' | 'running' | 'disposed'`. `AgentId` is a branded string. `AgentOptions` (`model?`, `systemPrompt?`) is merge-extensible — plugins add creation options by declaration merging. The `agent/*` event taxonomy (lifecycle, turn/step boundaries, the serial `agent/pre-step` surface-mutation seam, and the `agent/request`/`agent/step-result`/`agent/turn-continuation` waterfalls) is in [architecture.md § Event taxonomy](../architecture.md#event-taxonomy).
+`AgentStatus` is `'idle' | 'running' | 'disposed'`. `AgentId` is a branded string. `AgentOptions` (`model?`, `systemPrompt?`) is merge-extensible — plugins add creation options by declaration merging. The `agent/*` event taxonomy (lifecycle emits, the serial `agent/pre-step` surface-mutation seam, and the `agent/request`/`agent/step-result`/`agent/turn-continuation` waterfalls) is in [architecture.md § Event taxonomy](../architecture.md#event-taxonomy); turn/step boundaries are durable `session/event` records, not `agent/*` emits.
 
 ## `ToolDefinition`