fix: address PR #1802 review round

- listChildren reads the session store via strict ctx.get (property proxy
  is caller-scoped), orders candidates branchlessly, narrows the cold-read
  return type, and pins the cost model and store/registry composition gaps
  with tests; per-file coverage restored
- acp-agent and headless-agent compositions mount session-projection; a
  keyless snapshot pins the descriptor-less diagnostic row
- api-proxy cold spec pins header-origin ownership and the legacy
  descriptor-only opt-out
- design note ships as implemented with its English pairing; companion
  notes and core-data-structures pages synced

packages/subagent/tool-subagent-control/tests/list-agents.spec.ts

@@ -8,6 +8,7 @@ import AgentLoop from '@deepseek-ai/dsh-agent-loop'
 import { mountAgentLoopTestDependencies } from '@deepseek-ai/dsh-agent-loop-testkit'
 import { SessionId } from '@deepseek-ai/dsh-session'
 import JsonlSessionPersistence from '@deepseek-ai/dsh-session-persistence-jsonl'
+import SessionProjectionRegistry from '@deepseek-ai/dsh-session-projection'
 import SubagentService from '@deepseek-ai/dsh-subagent'
 import type { SubagentListEntry } from '@deepseek-ai/dsh-subagent'
 import * as SubagentSpawn from '@deepseek-ai/dsh-subagent-spawn'
@@ -28,6 +29,7 @@ async function setup(script: ConstructorParameters<typeof MockAdapter>[0]) {
   roots.push(root)
   await ctx.plugin(JsonlSessionPersistence, { root })
   await ctx.plugin(AgentLoop, { agents: [] })
+  await ctx.plugin(SessionProjectionRegistry)
   await ctx.plugin(SubagentService)
   await ctx.plugin(SubagentSpawn, { providerName: 'spawn' })
   await ctx.plugin(tool)