fix: address review round three

- the subagent projection's empty state becomes a serializable null
  sentinel (undefined fields vanish in JSON push frames, leaving stale
  identities in clients); consumers treat null and a missing key alike,
  and cached sentinels defer to the authoritative refold
- the tool catalog generator mounts the projection registry for
  list_agents; the shared unavailable-error copy goes catalog-neutral
- the design note's verification section records the new
  subagent-diagnostic snapshot honestly, the 2026-07-22 note's test
  inventory is rewritten to the current mechanism, and the session-store
  error code is documented

packages/host/apiproxy/tests/api-proxy-subagents.spec.ts

@@ -164,7 +164,7 @@ describe('subagent gateway', () => {
     )
     const expected = {
       code: 'internal',
-      message: 'subagent listing is unavailable: this deployment does not mount the sessionProjections registry (load @deepseek-ai/dsh-session-projection)',
+      message: 'subagent catalog is unavailable: this deployment does not mount the sessionProjections registry (load @deepseek-ai/dsh-session-projection)',
     }
 
     const list = bench({ listError: listError() })