Support other providers than Anthropic

[tomzx]

Users should be able to use any provider that can be accessed through Claude Code, for instance z.ai models or MiniMax models.

[github-actions]

I've created a plan for this issue. You can review it here: #9

Plan Summary

The plan proposes supporting multiple LLM providers (z.ai, MiniMax, and any OpenAI-compatible API) through the existing Claude Code GitHub Action by using provider-agnostic configuration variables.

Key Insight

The anthropics/claude-code-action already supports custom base URLs and models through environment variables, so we can support any OpenAI-compatible API without creating provider-specific workflows or GitHub Actions.

Main Changes:

1. Rename ANTHROPIC_BASE_URL → PROVIDER_BASE_URL
2. Rename ANTHROPIC_MODEL → PROVIDER_MODEL
3. Add backward compatibility fallback for existing users
4. Create provider setup documentation

This approach is backward compatible and requires no new GitHub Actions or major architectural changes.

Once you approve the plan by adding the plan-approved label, the implementation will begin automatically.