Add new `anthropic_base_url` config key

Details on the envvar: [https://code.claude.com/docs/en/env-vars#:~:text=ANTHROPIC_BASE_URL](https://code.claude.com/docs/en/env-vars#:~:text=workspace-id%20header-,ANTHROPIC_BASE_URL,-Override%20the%20API)

No file or test is named in the issue. Start by locating existing configuration keys and their environment-variable handling, then trace how the Claude Agent SDK client is configured. Done means the anthropic_base_url key is accepted and applied according to the linked ANTHROPIC_BASE_URL documentation, with relevant tests passing.

Written by the indexing model from the issue text.