feat: add findings artifact component

[juliarvalenti]

Summary

• Adds the findings artifact component for displaying key insights, recommendations, or discoveries with source attribution
• Based on contribution from @jaswantsingh09 in PR component: findings artifact #3, with pattern fixes applied

Fixes Applied

• findings.tsx: Refactored from 5 split UI components (hax/components/findings/) into a single self-contained file following standard artifact pattern
• types.ts: Added .describe() annotations on all Zod schema fields
• action.ts: Added Zod validation for findingsJson parsing
• index.ts: Added missing FINDINGS_DESCRIPTION barrel export
• Dependencies: Removed @radix-ui/react-popover dependency; replaced popover overflow with inline expand
• Registry: Simplified entry (no UI component registration needed); dropped duplicate files for already-merged artifacts

Files

hax/artifacts/findings/
├── findings.tsx     # Self-contained panel with cards, source chips, expand overflow
├── types.ts         # Zod schemas with .describe() annotations
├── action.ts        # CopilotKit hook with Zod validation
├── description.ts   # AI prompt guidance
├── index.ts         # Barrel exports
└── README.md        # Documentation

Test plan

• hax add artifact findings installs correctly
• Component renders with sample data and source chips
• Overflow "+N" chip expands to show all sources inline