Update agent to support model switching mid-session

[jbdamask]

Description

Modify the agent to allow changing LLM models during a session while preserving conversation context.

Requirements

• Agent tracks current model selection
• /model command triggers model switch
• Conversation history preserved across model changes
• Banner/UI shows current model name
• Graceful handling if API key for selected provider is missing

Implementation

• Add currentModel field to Agent struct
• Create model registry with provider mappings
• Update processTurn() to use current model's client
• Show model name in UI/banner

Related Issues

• Implement /model slash command for switching LLM models #18 - /model command
• Abstract LLM client interface for multi-provider support #19 - LLM client abstraction