feat: Add config system, agent mode, and session management
- Add comprehensive Config system with TOML support (~/.miyabi/config.toml)
- Add Agent module for autonomous task execution with tool approval
- Add Session management for conversation persistence
- Extend CLI with new commands: init, sessions, agent, version
- Add CLI flags: --model, --max-tokens, --thinking, --config, --session
- Fix all clippy warnings (16 → 0)
- Improve code quality with collapsible pattern matching
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Co-Authored-By: Claude <noreply@anthropic.com>
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