feat: Store multiple providers/models in btw.md

[gadenbuie]

Multiple Providers and Models

This PR updates btw.md to allow users to configure multiple client options in their btw.md file. When
btw_client() is called interactively from the console, you'll be presented with a menu to choose which client to use. In non-interactive contexts, the first client is used automatically.

Array format (unnamed list)

client:
  - anthropic/claude-sonnet-4
  - openai/gpt-4.1
  - aws_bedrock/us.anthropic.claude-sonnet-4-20250514-v1:0

Alias format (named list)

client:
  haiku: aws_bedrock/us.anthropic.claude-haiku-4-5-20251001-v1:0
  sonnet:
    provider: aws_bedrock
    model: us.anthropic.claude-sonnet-4-5-20250929-v1:0
  chatgpt: openai/gpt-5.2

With aliases, you can select a client by name in the interactive menu or pass the alias directly: btw_client(client = "sonnet").