TD-005: Unused ProviderMultimodalPayload Type

[murdore]

Summary

The ProviderMultimodalPayload type is not actively used by any provider implementations. Providers use SDK-specific types instead.

Root Cause

• Generic type created but providers use SDK-specific types
• Overly generic with [key: string]: unknown defeating type safety
• No migration path to remove unused types

Fix

Deprecate with comprehensive migration guide:

/**
 * @deprecated This generic type is not actively used by NeuroLink providers.
 * It will be removed in v9.0.0. Use provider-specific SDK types instead.
 * 
 * ## Migration Guide
 * - OpenAI: `import type { ChatCompletionMessageParam } from 'openai';`
 * - Anthropic: `import type { MessageParam } from '@anthropic-ai/sdk';`
 * - Google AI: `import type { Content } from '@google/generative-ai';`
 */

Acceptance Criteria

• [ ] @deprecated tag added
• [ ] Migration guide for 6+ providers
• [ ] Added to CHANGELOG.md
• [ ] Added to docs/MIGRATION.md
• [ ] Type still compiles (backward compatibility)

Files to Modify

• src/lib/types/multimodal.ts (lines 297-305)
• CHANGELOG.md
• docs/MIGRATION.md (create if needed)