feat: bullet points and visualizations for AI assessment sections

[CodeBuildder]

Summary

• Replace the agent system prompt with a plain-English JSON schema that includes bullet arrays
• Add what_happened, blast_radius_bullets, and action_steps to AgentDecision and audit incidents
• Render AI Assessment sections as bullet lists with visual blast-radius bars and numbered action steps
• Preserve fallback rendering for older incidents that do not have the new arrays yet

Checks

• .venv/bin/python -m pytest passed from agent/ with 71 tests
• npm run build passed from ui/
• UI build still emits existing non-blocking Vite/CSS warnings about CJS API deprecation, package module type, and @import ordering