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