feat: add contextual-explanation artifact

[juliarvalenti]

Summary

• Adds the contextual-explanation artifact component for displaying structured explanations of system changes, agent decisions, or automated actions
• Based on contribution from @AkankshaKhandelwal in PR Component: Contextual Explanation #6, with pattern fixes applied

Fixes Applied

• types.ts: Export ExplanationDetailZod with .describe() annotations; removed unnecessary ArtifactTabZod discriminated union
• action.ts: Added Zod validation for detailsJson parsing; use direct ContextualExplanationArtifact["data"] type
• index.ts: Added missing CONTEXTUAL_EXPLANATION_DESCRIPTION and ExplanationDetailZod exports
• Registry: Added alongside existing artifacts (original PR overwrote capability-manifest due to stale branch)

Files

hax/artifacts/contextual-explanation/
├── contextual-explanation.tsx   # Light-mode card with alert, details, 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 contextual-explanation installs correctly
• Component renders with sample data
• CopilotKit action create_contextual_explanation works with AI agents