refactor(events): remove the agent/steering mirror emit

agent/steering was the last transient mirror of a durable session event:
drainSteering appended the durable steering/message {turn, content, source}
and emitted the identical fact one line later. Zero production listeners
existed — every steering consumer (hook bridges, goldens, deriveMessages)
reads the durable event — and the one regression test subscriber asserted
a fact the log already records.

Remove the declaration (dsh-agent types + JSDoc list + README row), the
emit in drainSteering (its ctx parameter goes too), and the emit line in
the loop-pseudocode blocks (loop.ts module doc, architecture.md); the
cordis catalog is regenerated. The regression test now pins source
preservation on the durable steering/message event. Live-notification
needs keep their surviving homes: agent/queued at enqueue time,
session/event at drain time.

RFC: docs/rfc/implemented/simplification/2026-07-04-remove-agent-steering-mirror.md
(moved from proposed/, amended to shipped reality). The three implemented
RFCs that stated the retention — the boundary-mirror removal, the
stream-chunk removal, and event-domain-semantics — are amended to point
at that RFC as the record of the removal, per implemented/AGENTS.md.
The rejected retire-mid-turn-steering RFC keeps its frozen text (it
records the declined proposal); the steering capability itself —
steer(), the durable event, continuation forcing — is untouched.

packages/core/agent/src/types.ts

@@ -20,7 +20,7 @@
  *   `agent/request`/`agent/step-result`/`agent/turn-continuation` waterfalls and
  *   the serial `agent/pre-step`) that mutate/veto, and TRANSIENT emits
  *   (`agent/status`, `agent/error`, `agent/created`/
- *   `agent/disposed`, `agent/queued`, `agent/steering`, `agent/session-start`)
+ *   `agent/disposed`, `agent/queued`, `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
  *   object — intercept or observe."
@@ -339,12 +339,7 @@ declare module 'cordis' {
      */
     'agent/turn-continuation'(agent: Agent, turn: number, defaultDecision: ContinuationDecision, next: () => Promise<ContinuationDecision>): Promise<ContinuationDecision>
 
-    // ---- streaming + tool notifications (emit) ----
-    /**
-     * Steering content was injected into a running turn.
-     * @mode emit
-     */
-    'agent/steering'(agent: Agent, turn: number, content: ContentBlock[], source: MessageSource): void
+    // ---- error notifications (emit) ----
     /**
      * A step or turn errored. The loop reports a failure here (plus the logger)
      * even when the error has no in-turn position for a session `error` event.