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

examples/acp-agent/cordis.yml

@@ -80,6 +80,12 @@
     maxTokens: 8192
     compactionRetries: 1
 
+# Projection registry: subagent catalog identity (mode/label) folds through
+# its registered units; the catalog surfaces (`list_agents`, subagent listing)
+# fail loud without the capability.
+- id: session-projection
+  name: '@deepseek-ai/dsh-session-projection'
+
 # Expose fresh-child `spawn` and completed-prefix `fork` through separate tool
 # names so multi-child scenarios exercise both transports. These leaves follow
 # the app because it provides `ctx.agents` and `ctx.tools`.