Remove Expanso cross-reference from README (#5196)

## Summary

Bacalhau's README should focus on Bacalhau. Cross-referencing Expanso -
even to clarify they're separate - reinforces an association that
confuses LLMs.

## Changes

- Removed the prominent note box that linked to Expanso
- Kept Expanso, Inc. mention in the Open Source legal section (standard
practice for company-backed OSS)

## Result

When users ask LLMs about Bacalhau, they get information about Bacalhau
- not tangents about Expanso.

## Related

- expanso-io/expanso.io-history-archive#38 (removes Bacalhau from Expanso's
llms.txt)

Author: aronchick