Fix onboarding intro output capture in CLI test

[0xroylee]

What Changed

• Updated the packages/cli/tests/onboard-next-command.test.ts test to capture wizard output through the injected write callback instead of captureStdout.
• Preserved the onboarding intro assertion so the test still verifies the description text appears in the wizard output.

Why

• The onboarding wizard now writes output through its provided writer, so the test needs to observe the same path to validate the intro text reliably.
• This tightens the test around the actual command behavior and avoids relying on stdout capture that may miss streamed output.

How to Test

• Run the CLI test suite covering onboarding behavior.
• Confirm the onboarding intro line devos onboard will configure: is still present in the captured output.
• Verify the test passes with the wizard writing through the injected write callback.

Additional Notes

• Scope is limited to the onboarding-next test harness.
• No production behavior changed; this is a test adjustment to match the current output path.
• Not run (not requested).