Implement deep sleep / wake cycle

[xishell]

Description

Implement the ESP32-C3 deep sleep / RTC wake cycle. The node should sleep for a configurable interval (default 30 seconds), wake, power on the sensor, take readings, transmit status via ESP-NOW, then return to deep sleep.

Acceptance Criteria

• Node enters deep sleep and wakes on RTC timer
• Sleep interval is configurable (default 30s)
• Current draw during deep sleep is ~10 uA
• Full wake cycle (sensor on, read, transmit, sleep) completes within ~2 seconds