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

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.miyabi.yml

@@ -5,6 +5,12 @@ version: "0.1.0"
 # GitHub settings (use environment variables for sensitive data)
 # github_token: ${{ GITHUB_TOKEN }}
 
+# LLM Configuration
+llm:
+  provider: anthropic
+  model: claude-sonnet-4-20250514
+  api_key: ${{ ANTHROPIC_API_KEY }}
+
 # Agent settings
 agents:
   enabled: true