nixos: even with agent disabled settings.integration.agent.pre_authkey_path needs to be set otherwise headplane won't start #455

Description

Even if the agent is disabled and none of the settings.integration.agent options are set by the user, headplane still fails to start with the following:

2026-02-14T18:34:34.131Z [server] ERROR: Unable to load configuration: The configuration is missing required fields or has invalid values:
- integration.agent.pre_authkey must be a string (was missing)

Headplane Version

main (931a7f8)

Headscale Version

v0.28.0