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>
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