feat: add ReAct agent loop and build-time config (mimi_secrets.h)

Rewrite agent_loop.c with ReAct tool use loop: LLM call → tool
execution → repeat until end_turn (max 10 iterations). Add tool
guidance to system prompt. Add set_search_key CLI command.

Add mimi_secrets.h for build-time credentials with highest priority
over NVS/CLI values. All modules (wifi, telegram, llm, proxy,
web_search) check build-time secrets first, fall back to NVS.

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

main/mimi_secrets.h.example

@@ -0,0 +1,29 @@
+/*
+ * MimiClaw Build-time Secrets
+ *
+ * Copy this file to mimi_secrets.h and fill in your values.
+ * Non-empty values here take HIGHEST priority (override NVS/CLI).
+ * Leave empty ("") to use NVS values set via CLI.
+ *
+ *   cp mimi_secrets.h.example mimi_secrets.h
+ */
+
+#pragma once
+
+/* WiFi */
+#define MIMI_SECRET_WIFI_SSID       ""
+#define MIMI_SECRET_WIFI_PASS       ""
+
+/* Telegram Bot */
+#define MIMI_SECRET_TG_TOKEN        ""
+
+/* Anthropic API */
+#define MIMI_SECRET_API_KEY         ""
+#define MIMI_SECRET_MODEL           ""
+
+/* HTTP Proxy (leave empty or set both) */
+#define MIMI_SECRET_PROXY_HOST      ""
+#define MIMI_SECRET_PROXY_PORT      ""
+
+/* Brave Search API */
+#define MIMI_SECRET_SEARCH_KEY      ""