fix(agent-loop): append step/start before emitting agent/step-start (P1-6)

Every loop boundary appends the session event before emitting the Cordis
event (ADR 0003's append-before-emit rule) — except step/start, which was
inverted. A listener on agent/step-start that inspected session.events could
not see the step it was just told had started.

- Swap the two lines so session.append('step/start') precedes the emit.
- Fix the two stale pseudo-code copies (the runLoop JSDoc STEP-loop block and
  docs/architecture.md) so neither shows step-start emitted before the append.
- Regression test: a step-start listener observes the matching step/start
  event already at the tail of session.events. Verified the test fails on the
  pre-fix (emit-first) order.

docs/architecture.md

@@ -134,7 +134,7 @@ forever:
     drain queued → session('user/message'…) → 'turn/start' → emit agent/turn-start
     STEP loop:
       drain steering (late steering from previous step's listeners)
-      emit agent/step-start
+      session('step/start'); emit agent/step-start
       assembly = ctx.systemPrompt.assemble()          ⟵ waterfall system-prompt/assemble
       req = {model, system, tools, messages: session.deriveMessages(), signal}
       req = waterfall agent/request                   ⟵ hooks, compaction, model switch