feat: ${VAR} for API key configuration

[Winston-503]

Summary

This PR adds support for specifying API keys via environment variable references in configuration files using the ${VARIABLE} syntax.

Motivation

Previously, users could either hardcode API keys directly in config files (security risk) or rely on the implicit OPENAI_API_KEY fallback. This change allows explicit control over which environment variable to use, enabling:

• Different API keys for different models/providers
• Clear documentation of required environment variables in config
• Avoiding accidental key exposure in version-controlled config files

[CLAassistant]

All committers have signed the CLA.

[codelion]

Thanks for the PR! You're right that we need a way to reference environment variables in config files.

However, I'd prefer to use the more standard ${VAR} or $VAR substitution syntax that's common in config files (similar to Docker Compose, shell scripts, etc.). This would be more intuitive for users and follow existing conventions.

For example:

llm:
  api_key: ${OPENAI_API_KEY}
  models:
    - name: claude-sonnet-4-20250514
      api_key: ${ANTHROPIC_API_KEY}

This could be implemented by processing the loaded YAML dict and expanding any ${VAR} patterns using os.environ.get(). Would you be open to taking this approach instead?

[Winston-503]

I've updated the implementation to use the standard ${VAR} syntax as suggested. It's currently used for api_key only for simplicity. Recursive expansion for all config values (like api_base, etc.) could be added in the future if there's demand for it.