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

[z0w13]

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

[drifterza]

Yeah, you're not wrong about this @z0w13 as the config was checking for pre_authkey even when you had enabled: false.

PR #476 makes it conditional

• now it only requires that field when the agent is actually enabled.