fix(conversation): scope L0 search candidates by session

[YOMXXX]

Summary

• pass the optional sessionKey from tdai_conversation_search down into L0 store searches before candidate limiting
• add L0 search filter support to the store interface, SQLite FTS/vector paths, and TCVDB L0 hybrid search
• keep the existing tool-level post-filter as a defensive final guard

Root cause

executeConversationSearch accepted sessionKey, but searched global L0 candidates first and only filtered after FTS/vector results were already limited. If other sessions occupied the top candidates, the requested session could return no messages even when matching L0 rows existed.

Verification

• npx vitest run src/core/tools/conversation-search.test.ts (RED before fix, green after)
• npm test
• npm run build
• git diff --check

[Maxwell-Code07]

Awesome work! 13 fixes in one batch covering Gateway hardening, embedding compatibility, Hermes memory injection, offload stability, and pipeline optimization — a very productive cleanup session 👏

We'll review and merge them one by one 🚀