small improvements to claude

[AlexanderFengler]

This pull request introduces documentation updates and a new rule for Python package management. The main focus is on standardizing Python dependency management with uv and clarifying the project structure and release process in the documentation.

Python package management standardization:

• Added .claude/rules/uv.md to enforce uv as the package manager for all Python operations, specifying usage conventions and dependency management practices for Python files and pyproject.toml ([.claude/rules/uv.mdR1-R12](https://github.com/lnccbrown/HSSM/pull/940/files#diff-da931ef3887ed47461c1e5ed002463280b7e6415a13c73e22dcc6c42f0edd8bbR1-R12)).

Documentation improvements:

• Updated the project description in CLAUDE.md to clarify that HSSM is the user-facing package and to reference the broader HSSMSpine ecosystem ([CLAUDE.mdL5-R5](https://github.com/lnccbrown/HSSM/pull/940/files#diff-6ebdb617a8104a7756d0cf36578ab01103dc9f07e4dc6feb751296b9c402faf7L5-R5)).
• Documented the prepare-release.yml workflow in the CI overview table in CLAUDE.md, making the release preparation automation process more visible ([CLAUDE.mdR103](https://github.com/lnccbrown/HSSM/pull/940/files#diff-6ebdb617a8104a7756d0cf36578ab01103dc9f07e4dc6feb751296b9c402faf7R103)).