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.

docs/cordis-catalog/events-and-services.md

@@ -49,7 +49,7 @@ A step or turn errored. The loop reports a failure here (plus the logger) even w
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:353`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:348`](../../packages/core/agent/src/types.ts)
 
 #### `agent/pre-step` — serial
 
@@ -125,18 +125,6 @@ Types: [Agent](../core-data-structures/core.md)
 
 Source: [`packages/core/agent/src/types.ts:246`](../../packages/core/agent/src/types.ts)
 
-#### `agent/steering` — emit
-
-Steering content was injected into a running turn.
-
-```ts cordis-catalog
-'agent/steering'(agent: Agent, turn: number, content: ContentBlock[], source: MessageSource): void
-```
-
-Types: [Agent](../core-data-structures/core.md) · [ContentBlock](../core-data-structures/core.md) · [MessageSource](../core-data-structures/core.md)
-
-Source: [`packages/core/agent/src/types.ts:347`](../../packages/core/agent/src/types.ts)
-
 #### `agent/step-result` — waterfall
 
 Waterfall: post-process the assembled assistant Message before tool dispatch (validation, content rewriting, …).