Handle deletion of namespaces during dj push

[shangyian]

Summary

dj push ./nodes --namespace <ns> was supposed to sync the server-side namespace with the contents of the folder, but child namespaces were never pruned. If you deleted a sub-folder locally, the nodes inside it got removed but the empty namespace lingered on the server, breaking the "this folder is the source of truth" mental model during deployments.

This PR adds server-side namespace pruning in the deployment orchestrator. During plan creation, we diff the existing child namespaces under the deployment root against the desired set (built from local nodes' rendered names). Anything in the diff gets removed after node deletion, deepest-first. These changes are dry-run aware, so that calls to POST /deployments/impact also surface upcoming namespace deletions in its results.

This also adds a new result type: DeploymentResult.Type.NAMESPACE so that namespace operations show up distinctly in the deployment report alongside node / link / tag.

Test Plan

• PR has an associated issue: #
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

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