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.

packages/agent-loop/src/loop.ts

@@ -94,7 +94,7 @@ export interface LoopHandle {
  *     drain queued → session('user/message'…) → 'turn/start' → emit agent/turn-start
  *     STEP loop:
  *       drain steering → session('steering/message')  ⟵ catches late steering
- *       emit agent/step-start
+ *       session('step/start'); emit agent/step-start    ⟵ append before emit (ADR 0003)
  *       assembly = ctx.systemPrompt.assemble()        ⟵ waterfall system-prompt/assemble
  *       req = {model, system, tools, messages: session.deriveMessages(), signal}
  *       req = waterfall agent/request                 ⟵ hooks/compaction/model-switch
@@ -174,8 +174,8 @@ async function runTurn(ctx: Context, agent: LoopAgent, handle: LoopHandle, turn:
     // (or turn-start listeners on the first step) joins before the request.
     drainSteering(ctx, agent, turn)
 
-    ctx.emit('agent/step-start', agent, turn, step)
     session.append('step/start', { turn, step })
+    ctx.emit('agent/step-start', agent, turn, step)
 
     const abort = new AbortController()
     handle.setAbort(abort)