Improve resource clean-up after local agent is killed

[fitzn]

In #13 we added support for local execution of the agent. The happy path way to terminate the local agent is with Ctrl+C, and that change handles this signal properly by deleting and releasing resources such as processes. However, there might be other ways that the local agent execution terminates. In those unhappy paths, some resources such as the wireguard interface or the proxy process might be orphaned.

This tracks improving the robustness of the resource clean-up when the agent terminates unexpectedly.