Improve error messages when no API key is configured

[isaacsight]

What

When a user runs kbot for the first time without any API key configured, show a friendly, actionable setup guide instead of a raw error or stack trace.

Why

New users who install kbot via npm i -g @kernel.chat/kbot and immediately run kbot "hello" hit an authentication error. The current message points them to kbot auth but does not explain what they need (an API key from a provider) or give them options (free local models vs. cloud providers). A better first-run experience reduces friction and drop-off.

Suggested approach

1. In the main chat entry point in cli.ts (around line 3123 where the BYOK/auth check happens), detect when no provider is configured.
2. Instead of a one-line error, print a multi-line guide:

   No API key configured yet. Let's fix that!
   
   Quick start (pick one):
   
     Cloud (needs API key):
       kbot auth              Interactive setup — walks you through it
       kbot byok              Bring Your Own Key for 15+ providers
   
     Local (free, no key needed):
       kbot local             Use Ollama, LM Studio, or Jan
       kbot doctor            Check what's already installed
   
   Docs: https://github.com/isaacsight/kernel
   

3. Use the existing printInfo / printWarn helpers from ui.ts for consistent styling.
4. Exit cleanly (no stack trace).

Files to look at

• packages/kbot/src/cli.ts — Main entry point; the auth check before starting a chat session (~line 3123)
• packages/kbot/src/auth.ts — loadConfig(), isByokEnabled(), provider detection logic
• packages/kbot/src/ui.ts — printInfo, printWarn, printError helpers for styled output

[isaacsight]

Fixed in commit 51fa8b1. Closes #17.