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/rfc/implemented/simplification/2026-07-04-remove-agent-steering-mirror.md

@@ -0,0 +1,30 @@
+# RFC: Remove the `agent/steering` mirror emit
+
+Status: implemented (accepted 2026-07-04)
+
+## Problem
+
+`agent/steering` was the last remaining transient mirror of a durable session event. The loop's steering drain appends the durable `steering/message { turn, content, source }` and, on the very next line, emitted `agent/steering(agent, turn, content, source)` — the identical fact as a fire-and-forget event (`packages/core/agent-loop/src/loop.ts`, `drainSteering`). It had zero production listeners: the only subscriber anywhere was a loop regression test asserting the emit carried `source` — the same fact the durable event already records one line above.
+
+Both mirror-removal RFCs retained it while explicitly deferring the decision this RFC makes. The [boundary-mirror removal](2026-06-20-remove-agent-boundary-mirror-events.md) kept it as a live control signal rather than a boundary; the [stream-chunk removal](2026-07-02-remove-stream-chunk-mirror.md) retained it on the reading that it had no durable twin. The second rationale did not survive the code: the durable twin is `steering/message`, appended immediately before the emit with the same payload. The mirrored-vs-live-only line the taxonomy actually draws puts it on the mirror side: `agent/queued` is genuinely live-only (it fires at enqueue time, before any durable event exists, and already carries a `steering: boolean` flag — cancelled queued work never enters the log), while `agent/steering` fired at the exact moment its durable twin landed, carrying nothing the log does not.
+
+Steering carries real production traffic — the hook bridges' turn-continuation decisions inject their reasons through `inbox.steer()`, landing as durable `steering/message` events that the hook-matrix goldens pin — and every one of those consumers observes the durable event. Nothing observed the mirror.
+
+## Decision
+
+`agent/steering` is removed from the agent event taxonomy: the declaration in `packages/core/agent/src/types.ts` (and its mention in the live-events JSDoc list there), the emit in `drainSteering` (whose then-unused `ctx` parameter went with it), the row in `packages/core/agent/README.md`, and the emit line in the loop-pseudocode blocks (the `packages/core/agent-loop/src/loop.ts` module doc and [architecture.md](../../../architecture.md)); the cordis catalog is regenerated without it. The one regression test pins source preservation on the durable `steering/message` event — the fact it pins lives on the log.
+
+Three implemented RFCs stated the retention, and each is amended per [implemented/AGENTS.md](../AGENTS.md) to point here as the record of the removal: the [boundary RFC](2026-06-20-remove-agent-boundary-mirror-events.md)'s retained-list entry, the [stream-chunk RFC](2026-07-02-remove-stream-chunk-mirror.md)'s scope clause, and the [event-domain-semantics RFC](../architecture/2026-06-30-event-domain-semantics.md)'s transient-emit enumeration.
+
+## Why not keep it?
+
+"It is a control signal, not a boundary" — but the taxonomy's operative distinction is mirrored-vs-live-only, not control-vs-boundary, and this event mirrored. A consumer that wants enqueue-time notification has `agent/queued` (with its steering flag); a consumer that wants drain-time notification is by definition asking for the moment `steering/message` is appended, which `session/event` delivers with the same payload plus durability. The rejected [retire-mid-turn-steering RFC](../../rejected/simplification/2026-06-20-retire-mid-turn-steering.md) defended the steering *capability* — `steer()`, the durable event, continuation forcing — all of which this removal keeps untouched.
+
+## Acceptance criteria
+
+- The `agent/steering` spelling survives only in RFC prose (this RFC, the three amended RFCs above, and the frozen [rejected steering-capability RFC](../../rejected/simplification/2026-06-20-retire-mid-turn-steering.md), whose text records the proposal it declined); the catalog is regenerated and fresh.
+- The retargeted test pins source preservation on `steering/message`; the suite is green.
+
+## Risks
+
+None known: zero production listeners existed to migrate, and both live-notification needs (enqueue, drain) have surviving homes (`agent/queued`, `session/event`).