Add readonly session policy for helper and subagent sessions

[junyangyan4-svg]

Summary

• add a readonly session policy for subagent and helper sessions
• keep inherited recall available while skipping new long-term writes from ephemeral sessions
• block gm_record and gm_maintain in readonly sessions to make the policy explicit

Why

This isolates short-lived helper/subagent work from the shared graph so temporary execution chatter does not become long-term memory.

Split from #39.

Verification

• npm test
• npx tsc --noEmit

[adoresever]

方向有价值，但当前实现与“子代理可消费继承的召回上下文”这一目标冲突：before_prompt_build 对只读会话直接返回，context engine 又把 activeNodes 清空，因此子代理实际上也失去读取能力。请只阻止写入、提取和维护，不要阻止读取继承/既有记忆；同时补充测试覆盖：父会话召回传递给子会话、子会话不能写、结束后状态清理、普通会话不受 helper 名称启发式误伤。完成后可继续评审。