ci: delete runner caches when PR is closed

[Ankitsinghsisodya]

Problem

GitHub caps Actions cache storage at 10 GB per repository. Caches created during PR runs are scoped to refs/pull/<N>/merge and are never automatically removed when a PR is closed or merged. Over time, these orphaned caches consume the quota and cause evictions of useful caches on active branches.

Proposed Solution

Add a workflow triggered on pull_request: closed that lists and deletes all caches scoped to the closed PR's merge ref using gh actions-cache delete.

[Ankitsinghsisodya]

/assign