ci: extract reusable screenshot publish workflow

Move the full screenshot publishing pipeline into a new reusable workflow (`_publish-screenshots.yml`) and convert `publish-screenshots.yml` into a thin `workflow_run` wrapper that calls it. The reusable workflow adds inputs for artifact prefix, screenshot branch, and source run ID, and resolves workflow-run context via the Actions API so commit statuses, artifact downloads, screenshot syncing, and PR comparison comments all operate against the originating run.

Author: ReenigneArcher