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/support/ui-stdio/tests/ui-stdio.spec.ts

@@ -56,11 +56,19 @@ function makeAgent(id: string, status: AgentStatus = 'idle'): Agent & {
     status,
     sent,
     steered,
+    // A minimal session stub: the UI reads only `session.header.id` (to map the
+    // session back to its agent id for the turn-boundary label).
+    session: { header: { id: `${id}-session` } },
     send: (content: ContentBlock[]) => void sent.push(content),
     steer: (content: ContentBlock[]) => void steered.push(content),
   } as never
 }
 
+/** A session stub whose `header.id` matches an agent's, for `session/event` emits. */
+function makeSession(agentId: string): Session {
+  return { header: { id: `${agentId}-session` } } as Session
+}
+
 const CONFIG: Config = { welcome: 'hi there', agent: 'main' }
 
 async function setup(config: Config = CONFIG, runtimeOver: Partial<StdioRuntime> = {}) {
@@ -116,23 +124,54 @@ describe('createStdioChat rendering', () => {
     expect(out.text()).toBe(before)
   })
 
-  it('renders turn-start and turn-end markers', async () => {
+  it('renders turn/start and turn/end markers from the session feed', async () => {
     const { ctx, out } = await setup()
     const agent = makeAgent('main')
-    ctx.emit('agent/turn-start', agent, 3)
+    // agent/created populates the session-id → agent-id label map.
+    ctx.emit('agent/created', agent)
+    const session = makeSession('main')
+    ctx.emit('session/event', session, {
+      type: 'turn/start', seq: 1, time: 0, data: { turn: 3, trigger: { kind: 'message' } },
+    } as SessionEvent)
     expect(out.text()).toContain('[main turn 3] ')
-    ctx.emit('agent/turn-end', agent, 3, { kind: 'completed' })
+    ctx.emit('session/event', session, {
+      type: 'turn/end', seq: 2, time: 0, data: { turn: 3, reason: { kind: 'completed' } },
+    } as SessionEvent)
     expect(out.text()).toContain('\n> ')
   })
 
-  it('resets dim styling at turn-end if a turn ends mid-reasoning', async () => {
+  it('falls back to the session id as the label when no agent is mapped', async () => {
+    const { ctx, out } = await setup()
+    // No agent/created emitted, so the label map is empty — the header id shows.
+    ctx.emit('session/event', makeSession('orphan'), {
+      type: 'turn/start', seq: 1, time: 0, data: { turn: 1, trigger: { kind: 'message' } },
+    } as SessionEvent)
+    expect(out.text()).toContain('[orphan-session turn 1] ')
+  })
+
+  it('resets dim styling at turn/end if a turn ends mid-reasoning', async () => {
     const { ctx, out } = await setup()
     const agent = makeAgent('main')
     ctx.emit('agent/stream-chunk', agent, 1, 0, { type: 'reasoning-delta', index: 0, text: 'mid' })
-    ctx.emit('agent/turn-end', agent, 1, { kind: 'completed' })
+    ctx.emit('session/event', makeSession('main'), {
+      type: 'turn/end', seq: 1, time: 0, data: { turn: 1, reason: { kind: 'completed' } },
+    } as SessionEvent)
     expect(out.text()).toContain('\x1B[2mmid\x1B[0m')
   })
 
+  it('drops the label mapping on agent/disposed', async () => {
+    const { ctx, out } = await setup()
+    const agent = makeAgent('main')
+    ctx.emit('agent/created', agent)
+    ctx.emit('agent/disposed', agent)
+    // After disposal the map no longer resolves the agent id — fall back to the
+    // session header id.
+    ctx.emit('session/event', makeSession('main'), {
+      type: 'turn/start', seq: 1, time: 0, data: { turn: 1, trigger: { kind: 'message' } },
+    } as SessionEvent)
+    expect(out.text()).toContain('[main-session turn 1] ')
+  })
+
   it('renders tool/call and tool/result session events', async () => {
     const { ctx, out } = await setup()
     const session = {} as Session
@@ -196,7 +235,8 @@ describe('createStdioChat rendering', () => {
     const { ctx, out } = await setup()
     const before = out.text()
     ctx.emit('session/event', {} as Session, {
-      type: 'turn/start', seq: 1, time: 0, data: { turn: 1, trigger: { kind: 'continuation' } },
+      type: 'user/message', seq: 1, time: 0,
+      data: { content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' } },
     } as SessionEvent)
     expect(out.text()).toBe(before)
   })