fix(apiproxy): give a cold transcript read the agent its presenters live in

A preset registers its tools into the agent's OWN layer, so `session.history`
served while no agent exists found no presenter at all and every card degraded
to the generic renderer — silently, because a viewless entry is also what a
tool with no presenter produces. The web client opens a session by reading its
transcript, so this was the ordinary path, not an edge: the write row lost its
diff card, and with it the `locations` the produced-files row derives from.

The read now resolves the agent through the same deduplicated resume every
other session method takes, but only when a roster is composed: a deployment
without presets keeps its tools on the host layer, which needs no agent to
address, and keeps the storage-only read unchanged. A resolution failure stays
a successful read.

packages/host/apiproxy/tests/api-proxy-agent-preset.spec.ts

@@ -239,3 +239,37 @@ describe('a capability the session\'s preset mounts', () => {
     expect(failure.error.message).toContain('neither this session')
   })
 })
+
+describe('session.history presenters', () => {
+  it('resolves the live agent so a preset-composed presenter is reachable', async () => {
+    const { api, ctx } = await harness(['standard'])
+    await api.sessions.create(request({ sessionId: SessionId('h1'), agentPreset: 'standard' }))
+
+    const response = await api.sessions.history(request({ sessionId: SessionId('h1') }))
+
+    expect(response.result.ok).toBe(true)
+    expect(ctx.agents.get(SessionId('h1'))).toBeDefined()
+  })
+
+  it('serves the transcript when no agent can be resolved for it', async () => {
+    // The roster is composed, so the read tries; this harness resumes nothing.
+    // A resolution failure is not a read failure — the transcript still
+    // serves, with the generic cards a viewless entry renders.
+    const { api } = await harness(['standard'])
+
+    const response = await api.sessions.history(request({ sessionId: SessionId('h2') }))
+
+    expect(response.result.ok).toBe(false)
+    const failure = response.result as { ok: false; error: { code: string } }
+    expect(failure.error.code).toBe('session-not-found')
+  })
+
+  it('keeps the storage-only read when the deployment composes no roster', async () => {
+    const { api } = await harness()
+    await api.sessions.create(request({ sessionId: SessionId('h3') }))
+
+    const response = await api.sessions.history(request({ sessionId: SessionId('h3') }))
+
+    expect(response.result.ok).toBe(true)
+  })
+})