refactor: Replace AgentModel enum with unified Model struct

[ahmedhesham6]

Summary

This PR implements RFC 007 for dynamic model switching with a unified type system. It replaces the AgentModel enum (Smart/Eco/Recovery tiers) with a unified Model struct that contains all model metadata.

Changes Made

New Unified Model Type (libs/ai/src/types/model.rs)

• Model struct with fields: id, name, provider, reasoning, cost, limit
• ModelCost for pricing info (input/output per 1M tokens, cache pricing)
• ModelLimit for token limits (context window, max output)

Model Catalogs

• Added model definitions for Anthropic (libs/ai/src/providers/anthropic/models.rs)
• Added model definitions for OpenAI (libs/ai/src/providers/openai/models.rs)
• Added model definitions for Gemini (libs/ai/src/providers/gemini/models.rs)

API Changes

• AgentProvider trait methods now take Model instead of string/enum
• Added find_model() helper to search model catalog by ID
• Re-exported Model, ModelCost, ModelLimit from stakpak_api

TUI Updates

• Model switcher now works with Model struct
• Sidebar shows model name, provider, and token limits from Model
• Removed model label under input (info now in sidebar)

CLI Changes

• Removed --model CLI argument
• Model determined from config via get_default_model()
• Default model changed to Claude Opus 4.5

Cleanup

• Removed AgentModel enum entirely
• Removed ModelSet struct
• Removed unused parse_model, detect_provider methods
• Removed debug eprintln! statements

Testing

• All code compiles without errors
• No clippy warnings
• Code is formatted

Breaking Changes

• AgentModel enum removed - use Model struct instead
• --model CLI argument removed - configure model in config file