Bring back two-layer configuration: build-time defaults from mimi_secrets.h
with runtime NVS overrides via serial CLI. NVS values take highest priority
so a pre-flashed board can be reconfigured anywhere with just a USB cable.
Restored CLI commands: wifi_set, set_tg_token, set_api_key, set_model,
set_proxy, clear_proxy, set_search_key. Added new commands: config_show
(displays all config with sensitive fields masked), config_reset (clears
all NVS overrides), and help (lists all commands).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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