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/architecture.md

@@ -134,7 +134,7 @@ forever:
   wait for queued messages (idle)
   emit agent/status(running)
   TURN (error-contained — a throwing plugin ends the turn, never the loop):
-    drain queued → 'turn/start' → session('user/message'…) → emit agent/turn-start
+    drain queued → 'turn/start' → session('user/message'…)   ⟵ durable turn boundary (no agent/* mirror)
     STEP loop:
       drain steering (late steering from previous step's listeners)
       assembly = ctx.systemPrompt.assemble()          ⟵ waterfall system-prompt/assemble