E1.2.2 Add a concurrency regression test #30

Part of: T3 approval is process-wide state (not safe for concurrent or embedded use) (E1.2).

Task

Add a concurrency regression test

Subtasks

• Thread A signed off, thread B not; assert B is refused
• Assert the default after main() is unsigned

Definition of done

• Tests added or updated and CI green
• Docs updated when the command surface or output changes
• No private registry data, local paths or private repository names in fixtures or docs