feat: structured AI assessment with manager-friendly briefing

[CodeBuildder]

Summary

• Add plain-English writing guidance to the agent system prompt for manager-friendly assessments
• Structure the Threat Feed AI Assessment panel into What happened, Blast radius, and Recommended action sections
• Keep the design professional by using numbered section markers instead of emoji icons
• Preserve false-positive and confidence summary cards below the assessment sections

Design choices

• Used numbered markers (01, 02, 03) instead of decorative icons to keep the panel professional
• Kept severity-aligned colors for scanability: red for event context, orange for blast radius, blue for action

Checks

• docker build --no-cache -t argus-agent:latest . passed from agent/
• 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