fix: Check OAuth credentials before handler creation for GitHub Copilot

[Claude]

User successfully authenticated with --github-copilot-login but received "No OAuth credentials found" error when using GitHub Copilot models.

Root Cause

isApiKeyAvailable() checked envVar before oauthFallback. For GitHub Copilot (envVar: "", oauthFallback: "github-oauth.json"), empty string is falsy, causing early return true without verifying OAuth file exists.

Changes

• Reorder credential checks in isApiKeyAvailable(): Check oauthFallback first, return false if configured but file missing
• Apply fix to all 3 synced copies: src/, packages/core/src/, packages/cli/src/

function isApiKeyAvailable(info: ApiKeyInfo): boolean {
  // Check OAuth file first
  if (info.oauthFallback) {
    const credPath = join(homedir(), ".claudish", info.oauthFallback);
    return existsSync(credPath);  // Returns false if missing
  }
  
  // Then check envVar for non-OAuth providers
  if (!info.envVar) return true;  // Free tier
  // ...
}

Before: Handler created → credentials loaded → error thrown
After: Credentials checked → handler only created if available

Affects all OAuth-based providers (GitHub Copilot, Gemini Code Assist, Kimi).