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/subagent/src/index.ts

@@ -304,8 +304,8 @@ export class SubagentService extends Service {
    * @param signal - caller-owned cancellation forwarded to persistence reads
    *   and observed around every read await.
    * @returns children and per-child diagnostics ordered by `createdAt`, then id.
-   * @throws {@link SubagentError} when the projection registry is not mounted
-   *   or the caller cancels the listing.
+   * @throws {@link SubagentError} when the projection registry or the session
+   *   store is not mounted, or the caller cancels the listing.
    */
   listChildren(parentSessionId: SessionId, signal?: AbortSignal): Promise<SubagentListEntry[]> {
     return listSubagentChildren(this.ctx, parentSessionId, signal)