Optimize deployment preview pruning with sparse checkout

Context

Meshery's deployment preview workflow already has pruning logic that keeps preview deployments within a limit greater than 6. As a result, any PR preview older than that limit can be pruned and will only come back after a recent commit or push to that PR.

There is also a long-running step in the workflow that can likely be reduced by using sparse checkout instead of checking out the entire gh-pages branch.

Reference implementation

Meshery has a similar deployment preview fix that uses sparse checkout for preview deployment assets:

• https://github.com/meshery/meshery/blob/master/.github/workflows/build-and-preview-site.yml#L85-L87

Example of current waiting time

An example Meshery workflow run showing the long wait time:

• https://github.com/meshery/meshery/actions/runs/26299015072/job/77419065270

Proposed change

Apply the same Meshery deployment preview fix to Meshery's deployment preview workflow so that it checks out only the required preview deployment paths from gh-pages, rather than the full branch. This should reduce the time spent in the preview pruning/deployment step while preserving the existing preview pruning behavior.

Acceptance criteria

• Meshery deployment preview workflow uses sparse checkout for the relevant gh-pages preview deployment paths.
• Existing pruning behavior remains intact.
• Preview deployment step duration is reduced compared with full gh-pages checkout.