feat: tab to autocomplete the cycling agent name in onboarding

[shirshanka]

Summary

Small delight feature on the onboarding wizard.

Step 1 of the wizard ("Hello! I'm ___, your data analytics agent") already animates a typewriter cycling through a list of sample agent names — Aria, Nova, Scout, Sage, Atlas, Ember, Iris, Max, Luna, Felix, Zara, Rex. Now pressing Tab while the input is empty fills in whichever name is currently on screen. Spot a name you like as it cycles by, tap Tab, and it's yours.

What changes

A single keydown handler addition in OnboardingWizard.tsx:

```tsx
} else if (e.key === "Tab" && !value) {
e.preventDefault();
onChange(SAMPLE_NAMES[nameIdx]);
}
```

• Only fires when the input is empty — once the user has typed anything, Tab falls through to default focus behavior.
• Fills with the full current name (not the partially-typed prefix), so Tab mid-animation gives you the complete word, matching shell/editor autocomplete conventions.
• The existing typewriter `useEffect` already early-returns when `value` is truthy, so the carousel naturally stops after autocomplete.

Test plan

• Open onboarding wizard step 1, wait for a name to start typing, press Tab — input fills with the full name and the carousel stops.
• Type some text, then press Tab — focus moves to the next element (default behavior, autocomplete doesn't fire).
• Clear the input — carousel resumes.

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