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/src/index.ts

@@ -76,6 +76,15 @@ export function createStdioChat(ctx: Context, config: Config, runtime: StdioRunt
   const agentId = AgentId(config.agent ?? 'main')
   const { input, output, exit } = runtime
 
+  // Render label lookup: the `turn/start` session event carries only the turn
+  // number, so to print the short agent id (`[main turn 1]`) we map the
+  // session's id to its agent's id. The session id is not reliably the agent id
+  // (a session can be created with an explicit/client-supplied id), so build the
+  // map from `agent/created` rather than parsing the id string.
+  const labelBySession = new Map<string, string>()
+  ctx.on('agent/created', (agent) => { labelBySession.set(agent.session.header.id, agent.id) })
+  ctx.on('agent/disposed', (agent) => { labelBySession.delete(agent.session.header.id) })
+
   let inReasoning = false
   ctx.on('agent/stream-chunk', (_agent, _turn, _step, chunk) => {
     if (chunk.type === 'reasoning-delta') {
@@ -90,18 +99,18 @@ export function createStdioChat(ctx: Context, config: Config, runtime: StdioRunt
     }
   })
 
-  ctx.on('agent/turn-start', (agent, turn) => {
-    output.write(`\n[${agent.id} turn ${turn}] `)
-  })
-
-  ctx.on('agent/turn-end', () => {
-    if (inReasoning) output.write('\x1B[0m')
-    inReasoning = false
-    output.write('\n> ')
-  })
-
-  ctx.on('session/event', (_session, event) => {
-    if (event.type === 'tool/call') {
+  // Transcript rendering off the durable `session/event` feed — turn/step
+  // boundaries, tool activity, and todos all come from the one canonical stream
+  // (no agent/* boundary mirrors).
+  ctx.on('session/event', (session, event) => {
+    if (event.type === 'turn/start') {
+      const label = labelBySession.get(session.header.id) ?? session.header.id
+      output.write(`\n[${label} turn ${event.data.turn}] `)
+    } else if (event.type === 'turn/end') {
+      if (inReasoning) output.write('\x1B[0m')
+      inReasoning = false
+      output.write('\n> ')
+    } else if (event.type === 'tool/call') {
       const { name: toolName, arguments: args } = event.data
       if (inReasoning) output.write('\x1B[0m')
       inReasoning = false