fix(memos-local-plugin): ignore lightweight action vectors

[hijzy]

Summary

• stop embedding maintenance from treating lightweight memory trace action vectors as required slots
• keep lightweight trace summary vectors indexed and rebuildable
• add regression coverage for stats, repair, and rebuild behavior

Tests

• npm test -- tests/unit/pipeline/memory-core.test.ts
• npm test
• npm run lint
• npm run build

Notes

Lightweight memory intentionally stores only vec_summary. The previous maintenance stats counted the intentionally empty vec_action field as missing, so Settings reported missing embeddings for new lightweight chat memories even though retrieval had a summary index.