Move session summaries off blocking /new

[VACInc]

Summary

• move systemSessionMemory from the blocking command:new hook to async before_reset
• build the session summary from before_reset messages when available, with sessionFile fallback
• keep the stored session-summary memory format unchanged
• add focused coverage for the new async path and keep the hook registration regression check aligned

Why

systemSessionMemory currently does embedding + LanceDB writes inline on /new, which can delay ✅ New session started noticeably for larger sessions.

OpenClaw already exposes before_reset as a fire-and-forget hook with the previous session messages available, so this summary can still be stored without blocking /new.

Tradeoffs

• the session summary becomes eventually consistent instead of guaranteed before the first post-/new turn
• if something queries memory immediately after /new, that summary may not be available yet

Testing

• node --test test/session-summary-before-reset.test.mjs

[AliceLJY]

session summary 从同步 command:new 移到异步 before_reset，方向对，beta.10 的 OpenClaw hook 模型也更适合这种做法。eventual consistency 对 /new 场景可以接受。

麻烦 rebase 到最新 master，需要确认的测试：

1. 新的 test/session-summary-before-reset.test.mjs
2. test/plugin-manifest-regression.mjs 的 hook 注册更新
3. 手动跑一次 /new 确认延迟没有恶化

rebase + 测试过后可以合 👍

[VACInc]

Found a bug in OpenClaw preventing it from succeeding. All other tests passed except for on OpenClaw's side; I've opened a PR:
openclaw/openclaw#53872