feat(core): implement Tool trait and registry system

Add tool.rs with complete tool abstraction:
- Tool trait with async_trait for tool definitions
- ToolRegistry for managing and executing tools
- ParameterDef for declarative parameter schemas
- Automatic JSON schema generation
- Input validation before execution
- Integration with Anthropic tool format
- ToolOutput for structured results
- Comprehensive error types (NotFound, ValidationError, etc.)
- 12 unit tests covering registration, lookup, execution

Dependencies added: async-trait

Closes #24

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

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

Cargo.toml

@@ -22,6 +22,7 @@ categories = ["command-line-utilities", "development-tools"]
 # Async Runtime
 tokio = { version = "1", features = ["rt-multi-thread", "macros", "io-std", "sync", "process", "signal"] }
 futures = "0.3"
+async-trait = "0.1"
 
 # TUI Framework
 ratatui = { version = "0.29.0", features = [