Update config for ESPHome 2026.9, fix XIAO antenna switch and README

[firsttris]

Summary

Brings the config up to date with ESPHome releases since January 2026 and fixes several issues found in a review.

Config (esp32c6-thread-router.yaml)

• XIAO ESP32C6 RF switch: GPIO3 (switch power, LOW) and GPIO14 (antenna select) are now driven. Before, the external U.FL/SMA antenna of the recommended case was likely never used. Antenna is selectable via substitution antenna_default and an "External Antenna" switch in Home Assistant.
• Security: API encryption key, OTA uses encryption: {} (same key; ESPHome advises against an OTA password when API encryption is set).
• Diagnostics: openthread_info text sensors (role, RLOC16, channel, IP) and sensors (parent RSSI, TX retries, CCA errors, partition changes; new in 2026.8), plus uptime, restart button and status LED (GPIO15).
• Documented (commented out) force_dataset and output_power (new in 2026.3).
• name/friendly_name as substitutions, so additional routers are a small file that includes the main config via packages.

README

• The TLV contains the network key in plain text (was described as "encrypted"); added a warning.
• web.esphome.io can only flash, not compile; documented the compile + firmware.factory.bin flow.
• Explained REED → router promotion, force_dataset for recreated networks, transmit power, diagnostics table, antenna choice, new api_encryption_key secret.
• Consistent docker exec commands, new multi-router approach.

Other

• docker-compose.yml: relative mount instead of /home/tristan/..., removed test credentials, privileged and version; uses the stable tag and passes the USB device through. The config path in the container is now /config/esp32c6-thread-router.yaml.
• New CI workflow validating and compiling the config with dummy secrets (on push, PR and weekly).

Breaking change for existing devices

secrets.yaml needs a new api_encryption_key (openssl rand -base64 32), and Home Assistant will ask for it once after flashing.

Testing

• esphome config passes with ESPHome 2026.9.0 for the main file and a packages-based second-router file.
• A full compile could not be run locally (PlatformIO registry unreachable); the new CI workflow covers it.
• Not tested on hardware.

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