feat(agents): Show subagents as their own conversations (#132)

* feat(agents): Show subagents as their own conversations

Agents that support ACP subagent sessions (Claude Code, Codex) now run each
subagent as a child session. Its tool calls and messages no longer mix into
the task's conversation: the parent shows a card per subagent with its state,
tool call count, latest step, task and result, and the card opens the
subagent's own conversation.

- Route session/update by session id; subagent settings and usage stay out
  of the task.
- Subagent permission requests are answered in the task's conversation,
  named after the subagent.
- Subagent conversations are saved with the task; a running subagent is
  marked stopped after a restart.
- A running subagent can be stopped on its own.

* fix(agents): Save a subagent's state when it changes after the turn

src/platform/locales/text/zh.json

@@ -3208,6 +3208,17 @@
   "New worktree": "新 worktree",
   "Starting…": "正在启动…",
   "Waiting for your answer": "等待你的回答",
+  "Waiting for your answer…": "等待你的回答…",
+  "Subagent": "子 agent",
+  "Subagents at work": "工作中的子 agent",
+  "Stop Subagent": "停止子 agent",
+  "Open the subagent's conversation": "打开子 agent 的对话",
+  "Subagents take their instructions from the agent that started them.": "子 agent 只接收启动它的 agent 的指令。",
+  "Task from the parent agent": "父 agent 交给的任务",
+  "Report to the parent agent": "交给父 agent 的报告",
+  "Task": "任务",
+  "tool call": "次工具调用",
+  "tool calls": "次工具调用",
   "The agent stopped": "Agent 已停止",
   "Not installed": "未安装",
   "{command} was not found on PATH": "在 PATH 中找不到 {command}",