feat: add skills system with weather, daily-briefing, and skill-creator

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

main/skills/skill_loader.h

@@ -0,0 +1,20 @@
+#pragma once
+
+#include "esp_err.h"
+#include <stddef.h>
+
+/**
+ * Initialize skills system.
+ * Installs built-in skill files to SPIFFS if they don't already exist.
+ */
+esp_err_t skill_loader_init(void);
+
+/**
+ * Build a summary of all available skills for the system prompt.
+ * Lists each skill with its title and description.
+ *
+ * @param buf   Output buffer
+ * @param size  Buffer size
+ * @return Number of bytes written (0 if no skills found)
+ */
+size_t skill_loader_build_summary(char *buf, size_t size);