Add MiniMax provider support

[octo-patch]

Reason: add target provider/model to existing provider registry

Summary

• Register MiniMax as an OpenAI-compatible ClawSpring provider for MiniMax-M3.
• Add MiniMax-M3 context and pricing metadata, auto-detection, and README usage/API key documentation.

Checks

• python3 -m pytest tests/test_compaction.py tests/test_memory.py (blocked: pytest is not installed)
• python3 -m pip install -e '.[voice,vision]' pytest (blocked: externally managed Python environment)
• python3 -m venv /root/octopatch-4/work/venvs/recvoNmrOki5nQ && ... (blocked: python3.12-venv is unavailable; apt install failed due package signature/free-space errors)
• python3 -m py_compile providers.py
• inline import assertions for MiniMax provider detection, base URL, context, and cost
• secret scan with provided Octopatch regex
• git diff --check

[octo-patch]

This PR is superseded by #15, which has a clean commit history and complete target model, context, pricing, and endpoint coverage.