Fix explicit onboarding exit flow and onboarding viewport

[wangtsiao]

• Make explicit devo onboard exit after provider/model binding is saved, then print a focused next-step prompt to run devo.
• Keep first-run devo onboarding behavior unchanged: after onboarding completes, the user remains in the TUI.
• Hide the regular bottom composer/status line while onboarding is active.
• Keep the model-selection footer visible in short onboarding viewports.
• Ensure command exec notifications are subscribed for launched command exec requests and harden related tests.