add I2C and SPI tools for hardware interaction (#140)

* add I2C and SPI tools for hardware interaction

- Implemented I2CTool for I2C bus interaction, including device scanning, reading, and writing.
- Implemented SPITool for SPI bus communication, supporting device listing, data transfer, and reading.
- Added platform-specific implementations for Linux and stubs for non-Linux platforms.
- Updated agent loop to register new I2C and SPI tools.
- Created documentation for hardware skills, including usage examples and pinmux setup instructions.

* Remove build constraints for Linux from I2C and SPI tool files.

pkg/agent/loop.go

@@ -81,6 +81,10 @@ func createToolRegistry(workspace string, restrict bool, cfg *config.Config, msg
 	}
 	registry.Register(tools.NewWebFetchTool(50000))
 
+	// Hardware tools (I2C, SPI) - Linux only, returns error on other platforms
+	registry.Register(tools.NewI2CTool())
+	registry.Register(tools.NewSPITool())
+
 	// Message tool - available to both agent and subagent
 	// Subagent uses it to communicate directly with user
 	messageTool := tools.NewMessageTool()