Migrate from OpenRouter

Move OpenRouter code to BigModel.io by changing the base URL and key. Provider-prefixed model IDs and the models fallback array keep working.

Most OpenRouter code works on BigModel.io after two changes: the base URL and the API key.

OpenRouter feature	BigModel.io
`provider/model` IDs	Accepted; the provider prefix is removed
models fallback array	Supported; only the attempt that succeeds is charged
`HTTP-Referer` / `X-Title`	Supported
`provider`, `transforms`, `route`	Ignored
`:free` suffix	Not removed; use the plain ID from the directory

Change two lines

base_url = "https://bigmodel.io/v1"
api_key  = "sk-..."   # your BigModel.io key

Full migration guideModel ID mapping, headers, streaming and the differences to check before switching.