fix(sync): Improve Greenhouse kubeconfig loading and error reporting in sync command

[onuryilmaz]

Description

This PR improves the robustness and clarity of the sync command when loading the Greenhouse cluster kubeconfig. It addresses a common issue where client-go would report a cryptic "invalid configuration: no configuration has been provided" error when the default context in a kubeconfig file was missing or invalid, even if a specific context was provided via flags.

Key Changes

• Context-First Loading: If --greenhouse-cluster-context is specified, the tool now attempts to build the REST configuration using that context directly. This bypasses the initial attempt to load the default/current context, which was previously causing premature failures.
• Explicit File Validation: Added an explicit check for the existence of the Greenhouse kubeconfig file using os.Stat. This ensures a clear "file not found" error is returned instead of the generic Kubernetes configuration error.
• Enhanced Error Messages: Improved error wrapping to include the specific file path and context name that failed to load, making troubleshooting significantly easier for users.

Why this is needed

Previously, the sync command would call clientcmd.BuildConfigFromFlags("", path) which defaults to the "current-context". If the kubeconfig existed but the current-context was empty or pointed to a non-existent entry, the command would fail immediately, preventing the user-supplied --greenhouse-cluster-context from ever being used.