Clean up documentation and fix outdated references

- Remove internal tracking documents (AudioFixes.md, CODE_REVIEW.md)
- Remove unimplemented feature design doc (proxyAlbumArt.md)
- Update project structure refs: SendspinClient.Core → Sendspin.SDK
- Fix placeholder URLs (yourusername → chrisuthe)
- Add project structure diagram to CLAUDE.md with historical note
- Fix executable name consistency in USER_GUIDE.md

Author: chrisuthe