feat: add GPIO policy module and tool header

main/tools/tool_gpio.h

@@ -0,0 +1,27 @@
+#pragma once
+
+#include "esp_err.h"
+#include <stddef.h>
+
+/**
+ * Initialize GPIO tool — configure allowed pins and directions.
+ */
+esp_err_t tool_gpio_init(void);
+
+/**
+ * Write a GPIO pin HIGH or LOW.
+ * Input JSON: {"pin": <int>, "state": <0|1>}
+ */
+esp_err_t tool_gpio_write_execute(const char *input_json, char *output, size_t output_size);
+
+/**
+ * Read a single GPIO pin state.
+ * Input JSON: {"pin": <int>}
+ */
+esp_err_t tool_gpio_read_execute(const char *input_json, char *output, size_t output_size);
+
+/**
+ * Read all allowed GPIO pin states at once.
+ * Input JSON: {} (no parameters)
+ */
+esp_err_t tool_gpio_read_all_execute(const char *input_json, char *output, size_t output_size);