fix: use ANTHROPIC_AUTH_TOKEN to prevent login prompt

[0xCheetah1]

Summary

• Replace ANTHROPIC_API_KEY with ANTHROPIC_AUTH_TOKEN when spawning Claude Code in start.ts
• Claude Code checks ANTHROPIC_AUTH_TOKEN to determine if the user is authenticated, not ANTHROPIC_API_KEY
• Without this, Claude Code ignores the API key and shows a login prompt even though the proxy is running correctly

Problem

Running brcc start --model <model> would spawn Claude Code but it would prompt for login, even though:

• The proxy was running on the correct port
• Curl tests confirmed the endpoint worked

This happened because the previous code set ANTHROPIC_API_KEY which Claude Code ignores for auth detection.

Fix

Set ANTHROPIC_AUTH_TOKEN to a placeholder value (x402-proxy-handles-auth) since the proxy handles real authentication via x402/wallet signature:

env: {
  ...cleanEnv,
  ANTHROPIC_BASE_URL: `http://localhost:${port}/api`,
  ANTHROPIC_AUTH_TOKEN: 'x402-proxy-handles-auth',
  ANTHROPIC_DEFAULT_SONNET_MODEL: sonnetModel,
  ...
}

Test plan

• Run brcc start --model nvidia/nemotron-ultra-253b
• Confirm no login prompt appears and proxy routes correctly