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

docs/tool-catalog.md

@@ -32,7 +32,7 @@ This table connects model-visible tool names to the plugin package and service s
 | `@deepseek-ai/dsh-tool-skill` | `skill` | `ctx.tools`, `ctx.agents`, `ctx.skills` | `tool/call`, `tool/result`, `user/message replacement catalogs via agent.inject()` | - | - |
 | `@deepseek-ai/dsh-tool-session-query` | `session_event_read`, `session_event_search`, `session_event_trace`, `session_search`, `session_trace` | `ctx.tools`, `ctx.systemPrompt`, `ctx.sessionQuery`, `a calling Agent for workspace authority` | `tool/call`, `tool/result` | - | The five read-only tools hide provider cursors and authorize every result from the immutable calling agent session. The package is opt-in; compositions that need enforced deadlines or bounded inline output also mount the generic timeout or spill policies. |
 | `@deepseek-ai/dsh-tool-subagent` | `subagent` | `ctx.tools`, `ctx.subagents` | `tool/call`, `tool/result`, `child session events through the chosen provider` | `subagent`, `subagent_fork` | The registered tool name is the load-time `toolName` config (default `subagent`); the schema above is that default. The shipped example agents load this package once per subagent backend, so the model additionally sees `subagent_fork` (bound to the fork backend) with an identical schema — see `packages/bundle/base/cordis.patch.yml` and `examples/acp-agent/cordis.yml`. |
-| `@deepseek-ai/dsh-tool-subagent-control` | `list_agents`, `send_message` | `ctx.tools`, `ctx.subagents`, `ctx.sessionQuery (list_agents only)` | `tool/call`, `tool/result`, `child session events through ctx.subagents` | - | The globally named control tools over continuable background subagents: provider-bound `tool-subagent` instances register distinct delegation tools, while this package registers `send_message` once, plus `list_agents` from its separately loaded `/list-agents` plugin (which additionally requires session query). |
+| `@deepseek-ai/dsh-tool-subagent-control` | `list_agents`, `send_message` | `ctx.tools`, `ctx.subagents`, `ctx.sessionProjections (list_agents catalog rows)` | `tool/call`, `tool/result`, `child session events through ctx.subagents` | - | The globally named control tools over continuable background subagents: provider-bound `tool-subagent` instances register distinct delegation tools, while this package registers `send_message` once, plus `list_agents` from its separately loaded `/list-agents` plugin (whose catalog rows are served through the sessionProjections registry). |
 | `@deepseek-ai/dsh-tool-subagent-report` | `report` | `ctx.subagents`, `a live continuable in-process child Agent` | `tool/call`, `tool/result`, `a user-role message in the direct parent session` | - | Registered per continuable in-process child rather than globally, so this schema is visible only inside such a child and survives its global `toolFilter`. The parent-facing `send_message` tool is installed independently. |
 | `@deepseek-ai/dsh-tool-tasks` | `task_kill`, `task_list`, `task_output` | `ctx.tools`, `ctx.tasks`, `ctx.systemPrompt` | `tool/call`, `tool/result`, `user/message via agent.inject() for background completion notices` | - | The kind-agnostic background-task control surface: background bash commands, PTY sends, and subagents are read, listed, and killed through the same three tools. Loading the plugin attaches the control surface that arms producers' `ctx.tasks.start()`. |
 | `@deepseek-ai/dsh-tool-todo` | `todo_write` | `ctx.tools`, `owning Agent session` | `tool/call`, `todo/write`, `tool/result` | - | todo_write is session-owned state; UIs render the latest todo/write event as a checklist. |
@@ -1232,7 +1232,7 @@ Send a message to a background subagent by its subagent id, continuing the same
 
 Source: [`packages/subagent/tool-subagent-control/src/index.ts`](../packages/subagent/tool-subagent-control/src/index.ts)
 
-The globally named control tools over continuable background subagents: provider-bound `tool-subagent` instances register distinct delegation tools, while this package registers `send_message` once, plus `list_agents` from its separately loaded `/list-agents` plugin (which additionally requires session query).
+The globally named control tools over continuable background subagents: provider-bound `tool-subagent` instances register distinct delegation tools, while this package registers `send_message` once, plus `list_agents` from its separately loaded `/list-agents` plugin (whose catalog rows are served through the sessionProjections registry).
 
 ## `@deepseek-ai/dsh-tool-subagent-report`