feat: Support Claude subscription via CLIProxyAPI integration

[manurgdev]

Description

Users with a Claude Max or Pro subscription should be able to use Claude models in Magec without needing a separate API key. This can be achieved by integrating CLIProxyAPI, a local proxy that translates the subscription's OAuth credentials into a standard Anthropic-compatible API.

Motivation

Currently, using Claude in Magec requires an Anthropic API key, which means paying separately for API access on top of an existing subscription. Many users already pay for Claude Max/Pro and would prefer to reuse that subscription directly.

Proposed solution

1. Fix the Anthropic backend to honor the url field — currently it ignores it and always sends requests to https://api.anthropic.com, making it impossible to use any proxy or custom endpoint.
2. Add CLIProxyAPI as an optional service in Docker Compose, with its config and auth volume.
3. Integrate into the installer (install.sh) so users are prompted to enable the Claude subscription proxy during setup.
4. Document the setup in the backends and Docker Compose docs: backend configuration, OAuth login flow, and verification steps.

Acceptance criteria

• Anthropic backend respects the url field when configured
• CLIProxyAPI service available in Docker Compose deployments
• Interactive installer offers CLIProxyAPI as an option and preconfigures the backend
• Documentation covers end-to-end setup (login, backend config, verification)

[manurgdev]

Just opened a PR at #25

[achetronic]

Hello @manurgdev 🚀 Thank you for your contribution

The reason I have not covered this natively is because things like Anthropic getting mad about it: charmbracelet/crush#1783

But, with a Teams subscription, this is fair use according to the TOS, so your proposal is super nice. I will test it and run the AI team on top of my review to be sure that everything is aligned with the project.

Let's check it

[achetronic]

@manurgdev Revision done, I think we need to be more generic with the changes to cover this in a way we can extend it in the future. Could you perform the changes? I like the way this was covered in general 🙂