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/src/loop.ts

@@ -166,7 +166,7 @@ export interface LoopHandle {
  *                                                        → dispatch → tools/post-execute
  *         session('tool/result')
  *       append buffered post-execute additionalContext → session('context/message')(s)
- *       drain steering → session('steering/message'); emit agent/steering
+ *       drain steering → session('steering/message')
  *       session('step/end')                           ⟵ durable step boundary (no agent/* mirror)
  *       cont = waterfall agent/turn-continuation       ⟵ ContinuationDecision; default
  *         {action: hadToolCalls||steered ? 'continue':'stop'}; a continue.reason is
@@ -420,7 +420,7 @@ async function runTurn(ctx: Context, agent: ReactLoopAgent, handle: LoopHandle,
 
       // Steering from the previous round's continuation listeners joins before
       // the request.
-      drainSteering(ctx, agent, turn)
+      drainSteering(agent, turn)
 
       // The step's AbortController exists BEFORE any async pre-step work so a
       // dispose() or cancel() — in a synchronous turn-start listener or an
@@ -529,7 +529,7 @@ async function runTurn(ctx: Context, agent: ReactLoopAgent, handle: LoopHandle,
       if (stepReason) reason = stepReason
 
       // Steering that arrived during streaming/tool execution.
-      const steered = drainSteering(ctx, agent, turn)
+      const steered = drainSteering(agent, turn)
 
       if (closeStep()) break
 
@@ -635,11 +635,10 @@ async function runTurn(ctx: Context, agent: ReactLoopAgent, handle: LoopHandle,
 }
 
 /** Drain the steering queue into the session. Returns whether any arrived. */
-function drainSteering(ctx: Context, agent: ReactLoopAgent, turn: number): boolean {
+function drainSteering(agent: ReactLoopAgent, turn: number): boolean {
   const messages = agent.inbox.drainSteering()
   for (const message of messages) {
     agent.session.append('steering/message', { turn, content: message.content, source: message.source }, { surfaceOp: 'append' })
-    ctx.emit('agent/steering', agent, turn, message.content, message.source)
   }
   return messages.length > 0
 }

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' }])
   })
 })