Add MiniMax provider configuration via Anthropic-compatible env vars

[tars90percent]

Summary

• make Claude model selection configurable via MODEL with a safe fallback to claude-sonnet-4-5-20250929
• pass MODEL into the worker container environment in docker-compose.yml
• add concise README guidance for MiniMax as an Anthropic-compatible provider, including where to get a MiniMax API key and the required env vars (ANTHROPIC_API_KEY, ANTHROPIC_BASE_URL, MODEL)

Notes

• changes are surgical and limited to provider/model configuration and docs
• no Router mode changes included

[keygraphVarun]

#61