fix: remove unnecessary OAuth warning when API key auth is present (#77)

## 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](https://claude.com/claude-code)

<!-- CURSOR_SUMMARY -->
---

> [!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
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
f06b73f. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Co-authored-by: Claude Opus 4.5 <[email protected]>

Author: rgarcia

cmd/auth.go

@@ -88,7 +88,6 @@ func runAuth(cmd *cobra.Command, args []string) error {
 			} else {
 				pterm.Info.Printf("API Key: %s\n", strings.Repeat("*", len(apiKey)))
 			}
-			pterm.Warning.Println("Consider running 'kernel login' to use OAuth authentication")
 			return nil
 		}