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-loop/tests/review-fixes.spec.ts

@@ -410,7 +410,7 @@ describe('MEDIUM: misc registry and config fixes', () => {
     expect(agent.session.deriveMessages().at(-1)?.content).toEqual([{ type: 'text', text: 'routed' }])
   })
 
-  it('agent/queued carries the resolved source; agent/steering carries its source', async () => {
+  it('agent/queued carries the resolved source; steering/message records its source', async () => {
     const adapter = new MockAdapter([toolCallResponse('c1', 'noop', {}), textResponse('done')])
     const ctx = await harness(adapter)
     const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
@@ -425,15 +425,16 @@ describe('MEDIUM: misc registry and config fixes', () => {
     }))
 
     const queuedSources: { source: MessageSource; steering: boolean }[] = []
-    const steeringSources: MessageSource[] = []
     ctx.on('agent/queued', (_agent, _content, info) => void queuedSources.push(info))
-    ctx.on('agent/steering', (_agent, _turn, _content, source) => void steeringSources.push(source))
 
     send(agent, 'go') // no explicit source → default {kind:'user'} must be visible
     await waitForIdle(ctx, agent)
 
     expect(queuedSources[0]).toEqual({ source: { kind: 'user' }, steering: false })
     expect(queuedSources[1]).toEqual({ source: { kind: 'plugin', plugin: 'goal' }, steering: true })
+    // The drain appends the durable steering/message with the caller's source
+    // intact — the log, not a transient emit, is where consumers read it.
+    const steeringSources = agent.session.events.flatMap(e => e.type === 'steering/message' ? [e.data.source] : [])
     expect(steeringSources).toEqual([{ kind: 'plugin', plugin: 'goal' }])
   })
 })