fix: setup not working with custom provider

[Adnan-Najjar]

--setup writes llm.provider = "custom" without adding any
[custom_providers.<name>] entry, as the docs says and the generated
config fails at startup.

• When custom is selected, an extra question asks for the provider name
• build_config adds the named [custom_providers.<name>] entry with
  its api_key and base_url