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/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