Add design documents for studios: directive

- Add comprehensive design specification for studios directive
- Include Marimo notebook support as MVP approach
- Provide detailed implementation plan with file locations
- Design supports future extension to Jupyter, RMarkdown, and Seqera Studios

The studios directive will enable executing interactive notebooks (Marimo,
Jupyter, etc.) as Nextflow process steps, similar to Snakemake's notebook
feature.

Key decisions:
- Start with Marimo (pure Python, simpler than Jupyter)
- Map-based syntax following publishDir pattern
- JSON + CLI args for parameter passing
- Extensible architecture for future notebook types

Author: claude