feat(benchmarks): add MiniMax provider arm (MiniMax-M3, MiniMax-M2.7)

[octo-patch]

Reason: provider-add — add MiniMax benchmark coverage (MiniMax-M3, MiniMax-M2.7) via the OpenAI-compatible endpoint without changing the skill's prompt arms.

Changes

• Add benchmarks/promptfooconfig.minimax.yaml: a new promptfoo eval config that runs the existing baseline vs ponytail arms against MiniMax-M3 and MiniMax-M2.7 over the MiniMax OpenAI-compatible endpoint (https://api.minimax.io/v1). It reuses the repo's existing arms (arms/baseline.js, arms/ponytail.js) and the existing LOC + correctness gates (loc.js, correctness.js) on the same five everyday tasks as the other provider configs.
• Add MINIMAX_API_KEY to .env.example so the key is documented alongside the existing ANTHROPIC_API_KEY placeholder.
• Document the MiniMax run in benchmarks/README.md (a ### MiniMax reproduce section matching the Claude/Gemini/OpenAI sections).

The provider is wired as openai:chat:<model> with apiBaseUrl and apiKeyEnvar, so the API key stays in .env and no provider-specific promptfoo plugin is needed. No skill or prompt arm is touched; the new config is additive and mirrors the existing per-provider configs.

Checks run

• node --test tests/*.test.js — 82 passed, 0 failed.
• node --test benchmarks/correctness.test.js benchmarks/loc.test.js — passed.
• YAML parsed and validated with PyYAML (providers + tests shape correct).
• Secret scan on the added diff: clean.