refactor(events): remove the agent/stream-chunk mirror of assistant/chunk

The loop recorded every model token delta as a durable `assistant/chunk`
session event AND emitted an identical live `agent/stream-chunk` Cordis event
one line later. Same StreamChunk, same turn/step; the emit added only the live
Agent handle, which the sole consumer discarded. This is the boundary-mirror
duplication the event-domain work removed for turn/step boundaries, applied to
the token stream — a follow-up the boundary RFC explicitly deferred.

The premise is settled: chunk persistence is authoritative (the proposal to
stop persisting chunks was rejected — replay/snapshots depend on it), so
`assistant/chunk` on `session/event` is the load-bearing token stream and
`agent/stream-chunk` is pure redundancy.

- Remove the `agent/stream-chunk` declaration + emit; drop the now-unused
  StreamChunk import from dsh-agent's types.
- Migrate `dsh-ui-stdio` (the only live consumer; ACP already reads
  assistant/chunk off session/event) to render assistant/chunk in its existing
  session/event listener. Consolidating to one listener also makes the
  inReasoning dim-SGR flag deterministic across chunk/boundary events (they no
  longer race across two listeners).
- Repoint the agent-loop tests (cancel/loop) and ui-stdio tests to the
  session/event assistant/chunk feed.
- New RFC (implemented/simplification/2026-07-02-remove-stream-chunk-mirror);
  amend the boundary RFC's retained-list entry to cross-link; update
  architecture, cookbook, event-domain-semantics, the ACP proposal, and the
  regenerated cordis catalog.

Snapshot goldens unchanged (ACP never used the mirror), confirming no
editor-facing transcript change.

packages/core/agent-loop/src/loop.ts

@@ -158,7 +158,7 @@ export interface LoopHandle {
  *       req = {model, system, tools, messages: session.deriveMessages(), signal}
  *       req = waterfall agent/request                 ⟵ hooks/model-switch
  *       stream ctx.llm.stream(req)                    ⟵ waterfall llm/stream (raw chunks)
- *         session('assistant/chunk'); emit agent/stream-chunk
+ *         session('assistant/chunk')
  *       msg = waterfall agent/step-result             ⟵ BEFORE the log append, so the
  *       session('assistant/message' {content, usage?})   session records what actually ran
  *       each tool-call in msg (sequential, abort-checked):
@@ -681,7 +681,6 @@ async function runStep(
     if (signal.aborted) throw new Error(String(signal.reason ?? 'aborted'))
     const chunkEvent = session.append('assistant/chunk', { turn, step, chunk })
     chunkSeqs.push(chunkEvent.seq)
-    ctx.emit('agent/stream-chunk', agent, turn, step, chunk)
     assembler.push(chunk)
   }
 

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

@@ -176,7 +176,7 @@ describe('Agent.cancel()', () => {
     // 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 })
+    ctx.on('session/event', (_s, event) => { if (event.type === 'assistant/chunk') streamed = true })
     const dispose = ctx.on('session/event', (session, event) => {
       if (session === agent.session && event.type === 'turn/start') agent.cancel('from turn-start')
     })
@@ -205,7 +205,7 @@ describe('Agent.cancel()', () => {
     // cancel check (the one that must closeStep() to balance the already-open
     // step) — distinct from a turn-start cancel, caught before the step opens.
     let streamed = false
-    ctx.on('agent/stream-chunk', () => { streamed = true })
+    ctx.on('session/event', (_s, event) => { if (event.type === 'assistant/chunk') streamed = true })
     const dispose = ctx.on('session/event', (session, event) => {
       if (session === agent.session && event.type === 'step/start') agent.cancel('from step-start')
     })
@@ -245,7 +245,7 @@ describe('Agent.cancel()', () => {
 
     let disposalDone: Promise<void> | undefined
     let streamed = false
-    ctx.on('agent/stream-chunk', () => { streamed = true })
+    ctx.on('session/event', (_s, event) => { if (event.type === 'assistant/chunk') streamed = true })
     ctx.on('session/event', (session, event) => {
       if (session === agent.session && event.type === 'step/start') disposalDone = handle.dispose()
     })
@@ -308,7 +308,7 @@ describe('Agent.cancel()', () => {
     // runTurn. The second check (after the running flip) must drop the turn —
     // runTurn would otherwise throw on the now-empty queue.
     let streamed = false
-    ctx.on('agent/stream-chunk', () => { streamed = true })
+    ctx.on('session/event', (_s, event) => { if (event.type === 'assistant/chunk') streamed = true })
     const dispose = ctx.on('agent/status', (subject, status) => {
       if (subject === agent && status === 'running') agent.cancel('from running listener')
     })

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

@@ -137,21 +137,17 @@ describe('agent loop', () => {
     expect(request!.tools?.map(t => t.name)).toEqual(['noop'])
   })
 
-  it('records raw chunks for replay and emits agent/stream-chunk', async () => {
+  it('records raw chunks for replay as assistant/chunk session events', async () => {
     const adapter = new MockAdapter([textResponse('abc')])
     const ctx = await harness(adapter)
     const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
 
-    const streamed: StreamChunk[] = []
-    ctx.on('agent/stream-chunk', (_agent, _turn, _step, chunk) => void streamed.push(chunk))
-
     send(agent, 'hi')
     await waitForIdle(ctx, agent)
 
     const chunkEvents = agent.session.events.filter(e => e.type === 'assistant/chunk')
     // textResponse('abc') = block-start + 3 deltas + block-end + usage + finish = 7
     expect(chunkEvents).toHaveLength(7)
-    expect(streamed).toHaveLength(7)
     // replay: chunk events alone re-assemble to the recorded assistant message
     const deltaText = chunkEvents
       .flatMap(e => e.type === 'assistant/chunk' ? [e.data.chunk] : [])
@@ -685,10 +681,10 @@ describe('agent loop', () => {
     ctx.on('session/event', (_s, event) => { if (event.type === 'turn/start') turns.push(event.data.turn) })
 
     // queue two messages while idle — first starts turn 1 immediately;
-    // queue the second during turn 1 via a stream-chunk hook
+    // queue the second during turn 1 when the first assistant chunk streams
     let queued = false
-    ctx.on('agent/stream-chunk', () => {
-      if (!queued) {
+    ctx.on('session/event', (_s, event) => {
+      if (event.type === 'assistant/chunk' && !queued) {
         queued = true
         send(agent, 'second message')
       }

packages/core/agent/src/types.ts

@@ -19,7 +19,7 @@
  *   live `Agent`. Two shapes: INTERCEPTION seams (the `agent/prompt-submit`/
  *   `agent/request`/`agent/step-result`/`agent/turn-continuation` waterfalls and
  *   the serial `agent/pre-step`) that mutate/veto, and TRANSIENT emits
- *   (`agent/status`, `agent/stream-chunk`, `agent/error`, `agent/created`/
+ *   (`agent/status`, `agent/error`, `agent/created`/
  *   `agent/disposed`, `agent/queued`, `agent/steering`, `agent/session-start`)
  *   that notify with the `Agent` in hand. Turn/step boundaries are NOT here —
  *   they are durable `session/event` records. Answers "right now, with the agent
@@ -44,7 +44,7 @@
  */
 
 import type { Branded } from '@deepseek-ai/dsh-brand'
-import type { ContentBlock, GenerateOptions, Message, MessageSource, StreamChunk } from '@deepseek-ai/dsh-llm'
+import type { ContentBlock, GenerateOptions, Message, MessageSource } from '@deepseek-ai/dsh-llm'
 
 /** Identifies one live agent in the registry. */
 export type AgentId = Branded<'AgentId'>
@@ -340,11 +340,6 @@ declare module 'cordis' {
     'agent/turn-continuation'(agent: Agent, turn: number, defaultDecision: ContinuationDecision, next: () => Promise<ContinuationDecision>): Promise<ContinuationDecision>
 
     // ---- streaming + tool notifications (emit) ----
-    /**
-     * A raw {@link StreamChunk} arrived from the model (token-level UI/log feed).
-     * @mode emit
-     */
-    'agent/stream-chunk'(agent: Agent, turn: number, step: number, chunk: StreamChunk): void
     /**
      * Steering content was injected into a running turn.
      * @mode emit