* feat: add Korean stop words and tokenization for memory search
* fix: address review comments on Korean query expansion
* fix: lint errors - curly brace and toSorted
* fix(memory): improve Korean stop words and deduplicate
* Memory: tighten Korean query expansion filtering
* Docs/Changelog: credit Korean memory query expansion
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Co-authored-by: Vincent Koc <vincentkoc@ieee.org>
When searching in FTS-only mode (no embedding provider), extract meaningful
keywords from conversational queries using LLM to improve search results.
Changes:
- New query-expansion module with keyword extraction
- Supports English and Chinese stop word filtering
- Null safety guards for FTS-only mode (provider can be null)
- Lint compliance fixes for string iteration
This helps users find relevant memory entries even with vague queries.
