fix: remove unnecessary OAuth warning when API key auth is present

[rgarcia]

Summary

• Removes the warning that suggested running kernel login for OAuth when API key authentication is already present
• API key authentication via KERNEL_API_KEY is a valid authentication method that shouldn't prompt users to switch

Before:

kernel auth status
 INFO  Authentication method: API Key
 INFO  API Key: sk_cb5dc...SwTo
 WARNING  Consider running 'kernel login' to use OAuth authentication

After:

kernel auth status
 INFO  Authentication method: API Key
 INFO  API Key: sk_cb5dc...SwTo

Test plan

• Run kernel auth status with KERNEL_API_KEY set - verify no warning is shown
• Run kernel auth status without any auth - verify login instructions still appear

🤖 Generated with Claude Code

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Note

Removes unnecessary OAuth prompt when API key authentication is detected.

• In cmd/auth.go, eliminates the warning suggesting kernel login when KERNEL_API_KEY is set
• Unauthenticated flow still shows OAuth login instructions; OAuth status and token handling unchanged

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