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/tests/review-fixes.spec.ts

@@ -610,3 +610,32 @@ describe('HIGH: a finish-error stream chunk ends the turn as error, not complete
     expect(reasons).toEqual([{ kind: 'error', message: 'codeless failure' }])
   })
 })
+
+describe('P1-6: step/start is appended before agent/step-start is emitted', () => {
+  it('a step-start listener sees the step/start event already in session.events', async () => {
+    const adapter = new MockAdapter([textResponse('done')])
+    const ctx = await harness(adapter)
+    const agent = ctx.agentLoop.create('a-step-order', { model: 'mock' })
+
+    // Capture, at the moment agent/step-start fires, whether the matching
+    // step/start event is already in the log (append-before-emit, ADR 0003).
+    const observed: { turn: number; step: number; lastEventType: string | undefined; sawStepStart: boolean }[] = []
+    ctx.on('agent/step-start', (subject, turn, step) => {
+      if (subject !== agent) return
+      const events = [...subject.session.events]
+      const last = events.at(-1)
+      observed.push({
+        turn,
+        step,
+        lastEventType: last?.type,
+        sawStepStart: events.some(e => e.type === 'step/start' && e.data.turn === turn && e.data.step === step),
+      })
+    })
+
+    send(agent, 'go')
+    await waitForIdle(ctx, agent)
+
+    expect(observed).toHaveLength(1)
+    expect(observed[0]).toMatchObject({ turn: 1, step: 1, lastEventType: 'step/start', sawStepStart: true })
+  })
+})