Support OAuth authentication for Claude Pro/Max subscription usage

Problem

Currently, Claude Code requires an API key for authentication. Users with Claude Pro/Max subscriptions should be able to use Claude Code via OAuth, leveraging their existing subscription instead of paying separately for API usage.

Desired Behavior

Allow Claude Code to authenticate via OAuth so that users with active Claude Pro/Max subscriptions can use the tool under their subscription quota without needing a separate API key.

Motivation

• Reduces friction for subscribers who already pay for Claude access
• Avoids duplicate billing (subscription + API credits)
• Simplifies onboarding for non-developer users who may not be familiar with API key management