Personas: archetype field for unified agent profile

Added archetype as a structured field on the persona object. Apps can now store and retrieve the agent’s personality type (e.g. SAGE, EXPLORER) independently of the compiled system_prompt.

What changed:

• POST /personas and PUT /personas/{id} accept an optional archetype string (max 100 chars)
• All persona responses now include archetype (nullable)
• GET /personas/active returns archetype so apps can pattern-match to their own avatar assets without parsing prose

Why: TIE is the single source of truth for a user’s agent across all apps. archetype is the canonical identifier that lets any app read and render the agent correctly after onboarding in a different app. Avatar images are intentionally not stored in TIE — each app renders its own visual for the archetype.

Migration: No action required. Existing personas return archetype: null until updated.