feat: add Claude API proxy with SSE streaming parser

Anthropic Messages API integration with streaming response.
Parses content_block_delta SSE events, accumulates text tokens.
API key and model stored in NVS.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

main/llm/llm_proxy.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#include "esp_err.h"
+#include <stddef.h>
+
+/**
+ * Initialize the LLM proxy. Reads API key and model from NVS.
+ */
+esp_err_t llm_proxy_init(void);
+
+/**
+ * Send a chat completion request to Anthropic Messages API (streaming).
+ *
+ * @param system_prompt  System prompt string
+ * @param messages_json  JSON array of messages: [{"role":"user","content":"..."},...]
+ * @param response_buf   Output buffer for the complete response text
+ * @param buf_size       Size of response_buf
+ * @return ESP_OK on success
+ */
+esp_err_t llm_chat(const char *system_prompt, const char *messages_json,
+                   char *response_buf, size_t buf_size);
+
+/**
+ * Save the Anthropic API key to NVS.
+ */
+esp_err_t llm_set_api_key(const char *api_key);
+
+/**
+ * Save the model identifier to NVS.
+ */
+esp_err_t llm_set_model(const char *model);