fix: make agent pre_authkey optional when disabled

[drifterza]

Fixes #455

Problem

When using NixOS module with agent disabled, headplane fails to start because pre_authkey is required even when enabled: false.

Solution

Changed the agent config validation to only require pre_authkey when enabled is true. Uses arktype's narrow() for conditional validation.

Changes

• app/server/config/config-schema.ts - pre_authkey now optional with conditional check
• tests/unit/config/agent-disabled.test.ts - tests for disabled/enabled scenarios

Testing

• 4 new tests covering: disabled without key, enabled without key (should fail), enabled with key, no agent config

[tale]

I'm aware this was most likely written using AI given the unit test, but please remove this test, it is entirely unnecessary for what is a simple 5 line fix.

Edit: Either that or put it into existing config tests that we already have.

[drifterza]

Removed the separate test file as requested. The fix is straightforward - the conditional validation using .narrow() follows the same pattern used elsewhere in the codebase.

Changes: 9 lines in app/server/config/config-schema.ts only.