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>

.gitignore

@@ -27,6 +27,7 @@ mmap_generate_*.h
 
 # Environment
 .env
+main/mimi_secrets.h
 
 # Cache / Compiled
 .cache