feat: persona selector in viewer #34

Description

Add a persona selector toggle to the Evals and Ask pages. Users can switch between developer, architect, product-owner, and new-joiner perspectives to see how coverage and answers change per role.

Acceptance Criteria

• Persona dropdown/toggle in Evals page header
• Switching persona filters eval results to that persona's questions and scores
• Ask page optionally scopes answers to a persona's perspective
• Persona list reads from the eval config YAML (not hardcoded)
• Custom personas (from --persona-config) show up in the selector

Out of Scope

• Per-persona entity views in Explorer
• Persona-specific skill exports