Evaluate flashing spare Nest thermostats with NoLongerEvil firmware

## Summary

Investigate whether the spare Nest thermostats can be flashed with NoLongerEvil firmware for an experimental local Home Assistant setup.

## Context

The referenced discussion describes bringing supported Nest generations back into Home Assistant by flashing the thermostat, making a small configuration change, and connecting it through MQTT. The goal is experimentation with spare hardware, not replacing the currently used thermostat.

Reference: https://www.reddit.com/r/homeassistant/comments/1w13208/nolongerevilhome_assistant_and_10_minutes_now/

## Investigation tasks

- Identify the exact Nest thermostat generations and hardware available.
- Verify whether each model is supported by NoLongerEvil and document the supported firmware path.
- Confirm required flashing hardware, host operating-system limitations, recovery steps, and rollback options.
- Determine whether the workflow requires SSH, a local MQTT broker, or a Home Assistant add-on.
- Confirm that the experiment can be isolated from the active HVAC control path.
- Document risks to the thermostat, HVAC equipment, network, credentials, and existing automations.
- Define a safe validation plan using a spare thermostat before any production installation.
- Record the resulting entities, controls, telemetry, and limitations in Home Assistant documentation.

## Acceptance criteria

- A model-specific compatibility decision exists for each spare Nest thermostat.
- The flashing and recovery procedure is documented with prerequisites and a clear stop/rollback point.
- The experiment is isolated from live HVAC control unless explicitly approved.
- A working local Home Assistant/MQTT proof of concept is demonstrated, or incompatibility is recorded with evidence.
- No live thermostat is modified as part of the investigation without explicit approval.

Start by inventorying the spare Nest thermostat generations and reviewing the linked NoLongerEvil discussion for model support and firmware paths. Document flashing prerequisites, host limitations, recovery and rollback steps, isolation from live HVAC control, and the resulting Home Assistant/MQTT proof of concept or incompatibility evidence. Done means each model has a compatibility decision and a safe validation plan.

Written by the indexing model from the issue text.