feat: add ESP32 DHT22 MQTT sensor agent

[mingisrookie]

Summary

• replace the placeholder ESP32 loop with a configurable MicroClaw sensor firmware
• add DHT22 temperature/humidity readings with temperature and humidity alert thresholds
• add MQTT telemetry/status/alert publishing plus a command subscription for sample, sleep, and set_thresholds
• add optional timer deep sleep using esp_sleep_enable_timer_wakeup and esp_deep_sleep_start
• document wiring, build flags, MQTT topics, commands, and deep sleep behavior
• add static firmware contract tests so the ESP32/DHT22/MQTT/deep-sleep paths do not regress before hardware CI exists

Closes #2
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Closes #5

Bounty-board fit

This targets the MicroClaw ESP32 Port scope: ESP32 PlatformIO firmware, DHT22 driver, MQTT reporting/commands, deep sleep support, hardware/config docs, and verification coverage.

Testing

• python tests/test_firmware_static.py

Note: local PlatformIO was installed into an ignored .pio-venv, but pio run did not complete in this environment because package/toolchain resolution stalled; the PR includes the exact pio run command in README for hardware/toolchain CI.