feat: add non-streaming LLM API, tool registry, and web_search tool

Replace SSE streaming with non-streaming JSON for Anthropic API.
Add llm_chat_tools() returning structured llm_response_t with text
and tool_use blocks. Implement tool registry with dispatch-by-name
and web_search tool via Brave Search API (direct + proxy support).

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

main/tools/tool_web_search.h

@@ -0,0 +1,24 @@
+#pragma once
+
+#include "esp_err.h"
+#include <stddef.h>
+
+/**
+ * Initialize web search tool — loads API key from NVS.
+ */
+esp_err_t tool_web_search_init(void);
+
+/**
+ * Execute a web search.
+ *
+ * @param input_json   JSON string with "query" field
+ * @param output       Output buffer for formatted search results
+ * @param output_size  Size of output buffer
+ * @return ESP_OK on success
+ */
+esp_err_t tool_web_search_execute(const char *input_json, char *output, size_t output_size);
+
+/**
+ * Save Brave Search API key to NVS.
+ */
+esp_err_t tool_web_search_set_key(const char *api_key);