Add Merge Gateway as a first-class provider #2122

Summary

Add Merge Gateway as a first-class OpenAI-compatible gateway in OpenClaude. The provider is now published in models.dev and exposes a multi-provider catalog through Merge Gateway.

Proposed integration

• add a merge-gateway descriptor and /provider preset
• use https://api-gateway.merge.dev/v1/openai with MERGE_GATEWAY_API_KEY
• keep the credential dedicated to the Merge Gateway route
• support Chat Completions and Responses profiles
• discover the authenticated live model catalog with a cached static fallback
• document guided, CLI, and verification setup

Acceptance criteria

• Merge Gateway appears in provider setup and the web provider list
• saved profiles and --provider merge-gateway preserve the dedicated credential safely
• model discovery uses the OpenAI-compatible /models endpoint
• generated integration artifacts, focused tests, typechecks, build, and web validation pass

References: Merge Gateway docs, models.dev API.