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/tools/i2c.go

@@ -0,0 +1,147 @@
+package tools
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"path/filepath"
+	"regexp"
+	"runtime"
+)
+
+// I2CTool provides I2C bus interaction for reading sensors and controlling peripherals.
+type I2CTool struct{}
+
+func NewI2CTool() *I2CTool {
+	return &I2CTool{}
+}
+
+func (t *I2CTool) Name() string {
+	return "i2c"
+}
+
+func (t *I2CTool) Description() string {
+	return "Interact with I2C bus devices for reading sensors and controlling peripherals. Actions: detect (list buses), scan (find devices on a bus), read (read bytes from device), write (send bytes to device). Linux only."
+}
+
+func (t *I2CTool) Parameters() map[string]interface{} {
+	return map[string]interface{}{
+		"type": "object",
+		"properties": map[string]interface{}{
+			"action": map[string]interface{}{
+				"type":        "string",
+				"enum":        []string{"detect", "scan", "read", "write"},
+				"description": "Action to perform: detect (list available I2C buses), scan (find devices on a bus), read (read bytes from a device), write (send bytes to a device)",
+			},
+			"bus": map[string]interface{}{
+				"type":        "string",
+				"description": "I2C bus number (e.g. \"1\" for /dev/i2c-1). Required for scan/read/write.",
+			},
+			"address": map[string]interface{}{
+				"type":        "integer",
+				"description": "7-bit I2C device address (0x03-0x77). Required for read/write.",
+			},
+			"register": map[string]interface{}{
+				"type":        "integer",
+				"description": "Register address to read from or write to. If set, sends register byte before read/write.",
+			},
+			"data": map[string]interface{}{
+				"type":        "array",
+				"items":       map[string]interface{}{"type": "integer"},
+				"description": "Bytes to write (0-255 each). Required for write action.",
+			},
+			"length": map[string]interface{}{
+				"type":        "integer",
+				"description": "Number of bytes to read (1-256). Default: 1. Used with read action.",
+			},
+			"confirm": map[string]interface{}{
+				"type":        "boolean",
+				"description": "Must be true for write operations. Safety guard to prevent accidental writes.",
+			},
+		},
+		"required": []string{"action"},
+	}
+}
+
+func (t *I2CTool) Execute(ctx context.Context, args map[string]interface{}) *ToolResult {
+	if runtime.GOOS != "linux" {
+		return ErrorResult("I2C is only supported on Linux. This tool requires /dev/i2c-* device files.")
+	}
+
+	action, ok := args["action"].(string)
+	if !ok {
+		return ErrorResult("action is required")
+	}
+
+	switch action {
+	case "detect":
+		return t.detect()
+	case "scan":
+		return t.scan(args)
+	case "read":
+		return t.readDevice(args)
+	case "write":
+		return t.writeDevice(args)
+	default:
+		return ErrorResult(fmt.Sprintf("unknown action: %s (valid: detect, scan, read, write)", action))
+	}
+}
+
+// detect lists available I2C buses by globbing /dev/i2c-*
+func (t *I2CTool) detect() *ToolResult {
+	matches, err := filepath.Glob("/dev/i2c-*")
+	if err != nil {
+		return ErrorResult(fmt.Sprintf("failed to scan for I2C buses: %v", err))
+	}
+
+	if len(matches) == 0 {
+		return SilentResult("No I2C buses found. You may need to:\n1. Load the i2c-dev module: modprobe i2c-dev\n2. Check that I2C is enabled in device tree\n3. Configure pinmux for your board (see hardware skill)")
+	}
+
+	type busInfo struct {
+		Path string `json:"path"`
+		Bus  string `json:"bus"`
+	}
+
+	buses := make([]busInfo, 0, len(matches))
+	re := regexp.MustCompile(`/dev/i2c-(\d+)`)
+	for _, m := range matches {
+		if sub := re.FindStringSubmatch(m); sub != nil {
+			buses = append(buses, busInfo{Path: m, Bus: sub[1]})
+		}
+	}
+
+	result, _ := json.MarshalIndent(buses, "", "  ")
+	return SilentResult(fmt.Sprintf("Found %d I2C bus(es):\n%s", len(buses), string(result)))
+}
+
+// isValidBusID checks that a bus identifier is a simple number (prevents path injection)
+func isValidBusID(id string) bool {
+	matched, _ := regexp.MatchString(`^\d+$`, id)
+	return matched
+}
+
+// parseI2CAddress extracts and validates an I2C address from args
+func parseI2CAddress(args map[string]interface{}) (int, *ToolResult) {
+	addrFloat, ok := args["address"].(float64)
+	if !ok {
+		return 0, ErrorResult("address is required (e.g. 0x38 for AHT20)")
+	}
+	addr := int(addrFloat)
+	if addr < 0x03 || addr > 0x77 {
+		return 0, ErrorResult("address must be in valid 7-bit range (0x03-0x77)")
+	}
+	return addr, nil
+}
+
+// parseI2CBus extracts and validates an I2C bus from args
+func parseI2CBus(args map[string]interface{}) (string, *ToolResult) {
+	bus, ok := args["bus"].(string)
+	if !ok || bus == "" {
+		return "", ErrorResult("bus is required (e.g. \"1\" for /dev/i2c-1)")
+	}
+	if !isValidBusID(bus) {
+		return "", ErrorResult("invalid bus identifier: must be a number (e.g. \"1\")")
+	}
+	return bus, nil
+}