feat: add MiniMax as first SDK-based LLM provider

[octo-patch]

Summary

Adds MiniMax as the fifth LLM provider and the first to implement the SDK extension point (callSDK/callSimple). Unlike CLI-based providers (Claude, Codex, Gemini, Opencode), MiniMax uses a lightweight Node.js CLI wrapper that calls the MiniMax OpenAI-compatible API directly — no external CLI installation needed.

What is included

• Provider implementation (src/providers/minimax/): models, cli-builder, output-parser, cli-wrapper, SDK methods
• Models: MiniMax-M2.7 (1M context), M2.7-highspeed, M2.5, M2.5-highspeed (204K context)
• Level mapping: level1 -> M2.5-highspeed, level2 -> M2.7, level3 -> M2.7
• SDK support: getSDKEnvVar(), callSDK(), callSimple() — enables output reformatting when activated
• CLI wrapper: Node.js script that wraps the MiniMax API for the CLI-based orchestration pipeline
• Error classification: retryable (rate limits, server errors) vs permanent (invalid key, bad request)
• Provider registration: provider-names, capabilities, provider index
• Documentation: docs/providers.md and README.md updated
• Tests: 62 unit tests + 10 integration tests (all passing, 0 regressions)

Usage

export MINIMAX_API_KEY=your-key
zeroshot providers set-default minimax
zeroshot run 123

Design decisions

1. SDK-first provider: MiniMax has no dedicated CLI, so this implements the SDK extension point that BaseProvider documents. This paves the way for other API-only providers.
2. CLI wrapper for orchestration: The multi-agent pipeline uses buildCommand() -> spawn. The wrapper uses node + a bundled script to call the API and emit JSON events in the same format other providers use.
3. Temperature clamping: MiniMax accepts temperature in [0, 1], handled transparently.
4. Think-tag stripping: MiniMax reasoning models emit think tags which are stripped from output.

Test plan

• 52 unit tests for output parser, CLI builder, provider registration, model catalog, level mapping, error classification, SDK support, settings validation
• 10 integration tests for provider system integration, name normalization, settings, model resolution, CLI command generation
• All existing provider tests pass (87 total, 0 regressions)
• Manual: set MINIMAX_API_KEY and run zeroshot run with --provider minimax