refactor(events): remove the turn boundary mirror events

Complete the boundary-mirror removal begun with the step mirrors: drop
`agent/turn-start` and `agent/turn-end` from the agent event taxonomy. Turn and
step boundaries are now read exclusively off the durable `session/event` feed
(`turn/start`/`turn/end`/`step/start`/`step/end`) — there is no `agent/*` mirror
for any boundary.

- loop.ts: delete both turn emits; `closeTurn` loses its `emit` parameter and
  its now-unreachable idempotency guard (it is called exactly once per turn, on
  mutually exclusive normal/catch paths); `failTurn` loses the dead post-close
  branch that only a throwing turn-end LISTENER could reach.
- ui-stdio: render turn boundaries from `session/event`, recovering the short
  agent label from an `agent/created`→id map (the `turn/start` event carries only
  the turn number, and the session id is not reliably the agent id). ui-stdio is
  a disposable test REPL, so this migration retires the sole justification the
  event-domain-semantics RFC gave for KEEPING the turn mirrors.
- Tests: reason/turn-number collectors and the boundary-ordering test now read
  `session/event`; the throwing-turn-boundary-LISTENER tests are deleted (that
  code path no longer exists). A new test covers the outer-catch disposed branch
  via a pre-step listener that disposes-then-throws (the surviving real path).
- Docs: promote the "remove agent boundary mirror events" RFC to implemented
  (amended/narrowed — `agent/steering` is RETAINED, not a boundary mirror);
  update the event-domain-semantics + turn-enclosure RFCs, architecture.md, the
  cookbook, the ACP/agent/ui-stdio prose, and regenerate the cordis catalog.

`agent/steering` and `agent/stream-chunk` are explicitly out of scope (not
durable-boundary mirrors). ACP is unaffected — it already settles from the log's
`turn/end` + `agent/status`; snapshot goldens are byte-unchanged.

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

@@ -117,7 +117,7 @@ describe('Agent.cancel()', () => {
     const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
 
     const reasons: TurnEndReason[] = []
-    ctx.on('agent/turn-end', (_a, _t, reason) => void reasons.push(reason))
+    ctx.on('session/event', (_s, event) => { if (event.type === 'turn/end') reasons.push(event.data.reason) })
 
     send(agent, 'go')
     await new Promise(r => setTimeout(r, 30))
@@ -134,7 +134,7 @@ describe('Agent.cancel()', () => {
     const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
 
     const reasons: TurnEndReason[] = []
-    ctx.on('agent/turn-end', (_a, _t, reason) => void reasons.push(reason))
+    ctx.on('session/event', (_s, event) => { if (event.type === 'turn/end') reasons.push(event.data.reason) })
 
     send(agent, 'go')
     await new Promise(r => setTimeout(r, 30))
@@ -166,22 +166,23 @@ describe('Agent.cancel()', () => {
     expect(reasons.length).toBe(2)
   })
 
-  it('cancel from a synchronous agent/turn-start listener drops the step (step-start window)', async () => {
+  it('cancel from a synchronous turn/start session-event listener drops the step (step-start window)', async () => {
     const adapter = new MockAdapter([textResponse('should not stream')])
     const ctx = await harness(adapter)
     const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
 
-    // A turn-start listener fires BEFORE any AbortController is installed for the
-    // step. Cancelling there must still drop the step (the turn-scoped marker,
-    // not the step AbortController, is what catches this) — no model step runs.
+    // A turn/start listener fires right after turn/start is appended, BEFORE any
+    // AbortController is installed for the step. Cancelling there must still drop
+    // the step (the turn-scoped marker, not the step AbortController, is what
+    // catches this) — no model step runs.
     let streamed = false
     ctx.on('agent/stream-chunk', () => { streamed = true })
-    const dispose = ctx.on('agent/turn-start', (subject) => {
-      if (subject === agent) agent.cancel('from turn-start')
+    const dispose = ctx.on('session/event', (session, event) => {
+      if (session === agent.session && event.type === 'turn/start') agent.cancel('from turn-start')
     })
 
     const reasons: TurnEndReason[] = []
-    ctx.on('agent/turn-end', (_a, _t, reason) => void reasons.push(reason))
+    ctx.on('session/event', (_s, event) => { if (event.type === 'turn/end') reasons.push(event.data.reason) })
 
     send(agent, 'go')
     await waitForIdle(ctx, agent)
@@ -210,7 +211,7 @@ describe('Agent.cancel()', () => {
     })
 
     const reasons: TurnEndReason[] = []
-    ctx.on('agent/turn-end', (_a, _t, reason) => void reasons.push(reason))
+    ctx.on('session/event', (_s, event) => { if (event.type === 'turn/end') reasons.push(event.data.reason) })
 
     send(agent, 'go')
     await waitForIdle(ctx, agent)
@@ -271,9 +272,11 @@ describe('Agent.cancel()', () => {
     const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
 
     let steps = 0
-    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))
+    ctx.on('session/event', (_session, event) => {
+      if (event.type === 'step/start') steps += 1
+      if (event.type === 'turn/end') reasons.push(event.data.reason)
+    })
 
     let continued = false
     ctx.on('agent/turn-continuation', async (subject, _turn, _default, next) => {