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.

docs/architecture.md

@@ -147,7 +147,7 @@ forever:
       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')
       if assembler.finish is error/aborted: throw      ⟵ adapter's in-band error path →
                                                          step error (turn ends error/aborted,
                                                          not a normal completed message)
@@ -221,7 +221,7 @@ Every MVP feature (including the TODO-marked ones), with the mechanism that impl
 | Skills | section + tool registration; `inject()` skill content on invocation |
 | Memory | section provider + tool |
 | Scheduled tasks (cron) | plugin registers model-callable scheduling tools; timer fires → `send(…, {source: {kind: 'cron', …}})` when idle / `inject()` notification when busy |
-| UI (GUI; CLI emits JSONL) | listen `agent/stream-chunk` + `session/event`; input → `send()` |
+| UI (GUI; CLI emits JSONL) | listen `session/event` (assistant chunks, boundaries, tool activity); input → `send()` |
 | Telemetry / replayable trace | `session/event` → JSONL; replay = `sessions.create(id, { seed })` |
 | DeepSeek V4 (and other) models | `LlmAdapter` subclass via `registerAdapter`. **Implemented twice**: `dsh-llm-deepseek` (hand-rolled) and `dsh-llm-pi-ai` (pi-ai-backed) |
 | Plugin hot-reload | every registration is a `ctx.effect` → vendored HMR just works |