feat: add MiniMax as embedding provider (embo-01, 1536 dims)

[octo-patch]

Summary

Add MiniMax as a new embedding provider, supporting the embo-01 model (1536 dimensions).

MiniMax's embedding API uses a non-OpenAI-compatible format (uses texts array + type field for db/query distinction), so this implements a custom provider using native fetch rather than wrapping the OpenAI SDK.

Changes

• packages/core/src/embedding/minimax-embedding.ts — MiniMaxEmbedding class extending the base Embedding abstract class with full embed(), embedBatch(), detectDimension(), setModel(), setType(), and getSupportedModels() support
• packages/core/src/embedding/index.ts — Export the new provider
• packages/mcp/src/embedding.ts — Add MiniMax to the factory pattern and logging
• packages/mcp/src/config.ts — Add MiniMax to provider union type, config interface, env var reading, help text, and examples
• packages/mcp/README.md — Add MiniMax configuration section with setup instructions
• README.md — Add MiniMax to supported embedding providers list
• docs/getting-started/environment-variables.md — Add MINIMAX_API_KEY and MINIMAX_BASE_URL env vars
• .env.example — Add MiniMax configuration section

Configuration

EMBEDDING_PROVIDER=MiniMax
MINIMAX_API_KEY=your-minimax-api-key
# Optional: MINIMAX_BASE_URL=https://api.minimax.io/v1
# Optional: EMBEDDING_MODEL=embo-01

Key Design Decisions

• Custom fetch implementation instead of OpenAI SDK wrapper, because MiniMax's embedding API differs:
  • Request uses texts array (not input)
  • Request includes type field (db for indexing, query for search)
  • Response uses vectors array (not data[].embedding)
• type parameter support — setType('db' | 'query') allows switching between storage and query embedding modes, matching MiniMax's API semantics
• No new dependencies — Uses native fetch (available in Node.js 18+)

Test Plan

• 19 unit tests covering constructor, embed, embedBatch, detectDimension, setModel, setType, error handling, auth headers, baseURL routing, and text preprocessing
• 3 integration tests (require MINIMAX_API_KEY env var, auto-skipped without it)
• TypeScript builds cleanly for both packages/core and packages/mcp
• All existing provider interfaces and patterns preserved