[FEAT cloudctl] - Add set and env commands for persistent session config

[onuryilmaz]

User Story

As a cloudctl user, I want to set a default context, namespace, and kubeconfig path once and have all subsequent commands use them, so I don't repeat flags on every invocation.

Description

cloudctl set --context <ctx> --namespace <ns> --kubeconfig <path> persists config to the XDG config dir. cloudctl env prints evaluable shell output (CLOUDCTL_CONTEXT=, CLOUDCTL_KUBECONFIG=, CLOUDCTL_NAMESPACE=) for use in scripts and eval. Flags on individual commands override persisted config.

Acceptance Criteria

• set accepts --context, --namespace, --kubeconfig flags and persists to XDG config
• All cloudctl commands read persisted config as defaults
• env prints evaluable shell output for all three values
• Per-command flags override persisted config

Reference Issues

Epic: #70

[github-actions]

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