Add support for `protocol:unregister` and safe removal of deep link handlers

[pugsley76]

Description:
Glassbox should expose a command to cleanly unregister the custom glassbox:// deep link handler.

Work to be done:
Implement a protocol:unregister command that removes registered handler entries safely.

Implementation procedure:

1. Add the unregister command to the protocol command set.
2. Implement platform-specific removal of protocol handler registration.
3. Ensure the command handles missing or partially registered states gracefully.
4. Add tests for unregister behavior.
5. Document the command.

Acceptance criteria:

• There is a command to unregister the deep link handler.
• It removes registration entries cleanly.
• Tests cover removal and partial state handling.
• Documentation explains unregister usage.

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