fix(l1): restore extraction quality gates

[YOMXXX]

Summary

• keep L0 archival permissive while restoring stricter L1 extraction filtering
• reject trivial acknowledgements/short greetings before invoking the L1 extractor
• reject oversized pasted messages before they enter the LLM extraction path

Refs #82

Verification

• npm test -- src/utils/sanitize.test.ts
• npm test
• npm run build

[Maxwell-Code07]

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