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/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')
     })