Configure Agent.hs and Ava.hs to use Claude-code provider by default

Both Omni/Agent.hs and Omni/Ava.hs should default to using the Claude-code provider and token. This should use the Claude max account which is the cheapest and highest quality option.

Additionally, add a /provider command to the Telegram interface to allow switching providers at runtime.

Implementation considerations:

• Update default provider configuration in both Agent.hs and Ava.hs
• Ensure Claude-code credentials are properly referenced
• Add /provider telegram command for runtime provider switching
• Consider what the /provider command should display (current provider, available options, how to switch)