Improve error messages for missing API keys

[xmaks82]

Description

When a user tries to use a model without the required API key, the error message should be more helpful.

Current behavior

Generic error or traceback when API key is missing.

Expected behavior

Clear message like:
```
Error: GROQ_API_KEY is not set.

To fix this:

1. Get a free key at https://console.groq.com/
2. Add it to your .env file: GROQ_API_KEY=gsk_...
3. Or set it: export GROQ_API_KEY=gsk_...

Available providers with configured keys: SambaNova, Cerebras
```

Files to modify

• `src/agent/clients/factory.py` — create_client()
• `src/agent/main.py` — check_api_keys()