fix: add CLAUDE_CODE_USE_GEMINI to is3P check to prevent login screen

[gnanam1990]

Problem

Gemini users see the Anthropic/gcloud login screen at startup even when CLAUDE_CODE_USE_GEMINI=1 and GEMINI_API_KEY are correctly set. Reported in #43.

Root Cause

isAnthropicAuthEnabled() in src/utils/auth.ts uses an is3P check to decide whether to skip Anthropic auth. CLAUDE_CODE_USE_OPENAI was included but CLAUDE_CODE_USE_GEMINI was missing:

const is3P =
  isEnvTruthy(process.env.CLAUDE_CODE_USE_BEDROCK) ||
  isEnvTruthy(process.env.CLAUDE_CODE_USE_VERTEX) ||
  isEnvTruthy(process.env.CLAUDE_CODE_USE_FOUNDRY) ||
  isEnvTruthy(process.env.CLAUDE_CODE_USE_OPENAI)
  // CLAUDE_CODE_USE_GEMINI missing ← bug

With is3P = false, shouldDisableAuth = false, so isAnthropicAuthEnabled() returns true → login screen appears.

Fix

One line — add CLAUDE_CODE_USE_GEMINI to the is3P check, consistent with CLAUDE_CODE_USE_OPENAI.

Test

CLAUDE_CODE_USE_GEMINI=1 GEMINI_API_KEY=your-key openclaude
# Before: Anthropic/gcloud login screen appears
# After: starts directly, no login prompt

Closes #43.