Add persona-driven content generation experiments

[jeffpaul]

Introduce the concept of personas or voices that influence AI-generated content. Personas may represent roles, audiences, or brand voices and should be reusable across experiments.

This should be implemented in a way that allows future extension by plugins or themes.

Acceptance criteria:

• Ability to define and select personas for content generation
• Persona selection consistently influences output style and tone
• Personas are reusable across multiple experiments
• Architecture supports extension and customization