feat: initialize ESP32 project with PlatformIO

## Description
Set up the initial embedded project structure for MicroClaw using PlatformIO (ESP32).

## Requirements
- [ ] platformio.ini with ESP32 board configuration
- [ ] src/main.cpp with basic WiFi + MQTT setup
- [ ] lib/ directory for sensor drivers
- [ ] .gitignore for PlatformIO build artifacts
- [ ] README with flash instructions

## Skills
C/C++, PlatformIO, ESP32