refactor(events): document event-domain semantics, drop step-boundary mirror emits

Pin the three-domain rule (session = durable fact log, agent = live runtime
surface, tools = registry/exec): a durable replayable fact is a SessionEvent; a
live interception or transient/live-object signal is an agent/tools Cordis
event. A boundary that is both is mirrored as an agent/* emit ONLY where a live
consumer needs the Agent handle.

Apply it to the boundary twins: drop agent/step-start and agent/step-end (no
production consumer needs the live Agent at a step boundary — consumers read the
durable step/start/step/end session events). Keep agent/turn-start/turn-end (the
stdio UI labels output by agent.id). Tests that observed step boundaries via the
removed emits now observe the durable session events; the pinned behavior is
unchanged.

Conservative subset of the proposed "remove boundary mirror events"
simplification; foundation for the Hooks subsystem's canonical event surface.

packages/core/agent-loop/tests/cancel.spec.ts

@@ -204,7 +204,7 @@ describe('Agent.cancel()', () => {
     const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
 
     let steps = 0
-    ctx.on('agent/step-start', () => { steps += 1 })
+    ctx.on('session/event', (_session, event) => { if (event.type === 'step/start') steps += 1 })
     const reasons: TurnEndReason[] = []
     ctx.on('agent/turn-end', (_a, _t, reason) => void reasons.push(reason))