Integration test: config get, list, and delete

[cicalese]

Test canasta config get, canasta config get KEY, and canasta config delete KEY against a running instance.

The existing backup test exercises config set but not the get/list/delete paths, which have their own key resolution, multi-key output formatting, side-effect unapplication, and restart logic.

Estimated coverage gain: ~58 new statements in cmd/config/get.go and cmd/config/unset.go.

Partial progress on #241.

[cicalese]

Done in #734.