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

pnpm-lock.yaml

@@ -418,6 +418,9 @@ importers:
       '@deepseek-ai/dsh-session-persistence-jsonl':
         specifier: workspace:*
         version: link:../packages/session-persistence/session-persistence-jsonl
+      '@deepseek-ai/dsh-session-projection':
+        specifier: workspace:*
+        version: link:../packages/session-projection/session-projection
       '@deepseek-ai/dsh-session-query':
         specifier: workspace:*
         version: link:../packages/session-query/session-query
@@ -5587,6 +5590,9 @@ importers:
       '@deepseek-ai/dsh-session-persistence-jsonl':
         specifier: workspace:^
         version: link:../../session-persistence/session-persistence-jsonl
+      '@deepseek-ai/dsh-session-projection':
+        specifier: workspace:^
+        version: link:../../session-projection/session-projection
       '@deepseek-ai/dsh-subagent':
         specifier: workspace:^
         version: link:../subagent