feat: add diagnostic-report artifact component

[juliarvalenti]

Summary

• Adds the diagnostic-report artifact for presenting structured diagnostic findings with confidence levels and actionable recommendations
• Based on contribution from @yashwant-intelligaia in PR feat: add diagnostic report artifact #4, with pattern fixes applied

Fixes Applied

• types.ts: Fixed import { z } (was default import); exported DiagnosticItemZod with .describe() annotations; removed unnecessary ArtifactTabZod discriminated union
• action.ts: Added Zod validation for itemsJson parsing; use direct DiagnosticReportArtifact["data"] type instead of Extract<ArtifactTab, ...>
• index.ts: Added missing DIAGNOSTIC_REPORT_DESCRIPTION and DiagnosticItemZod exports; simplified exports
• Registry: Added alongside existing artifacts (original PR overwrote capability-manifest due to stale branch)

Files

hax/artifacts/diagnostic-report/
├── diagnostic-report.tsx   # Table layout with confidence bars, action buttons
├── 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 diagnostic-report installs correctly
• Component renders with sample diagnostic items and confidence bars
• CopilotKit action create_diagnostic_report works with AI agents