CI: delete a branch's Actions caches when the branch is deleted #476

The Actions cache sits at its 10 GB limit, and most of it can never be restored.

On 2026-10-26 the repository held about 7.4 GB in 40 listed caches (the usage endpoint reported 10.7 GB in 51, it lags). 6.2 GB in 21 of them belongs to branches that no longer exist. Only 1.2 GB belongs to live branches.

Almost all of it is Linux-nuget-<hash>, 270–300 MB each, written by GitHub's Automatic Dependency Submission run on every push to every branch. None of our workflows cache anything. A cache is scoped to its branch, and delete_branch_on_merge is on, so the cache of every merged PR's branch is dead weight until the 7-day idle expiry. Because of that, eviction at the 10 GB limit also pushes out caches that live branches still use.

Turning dependency submission off is not the fix. It is what puts the NuGet dependencies (Central Package Management) into the dependency graph that Dependabot alerts read.

Change

A workflow on main that deletes a branch's caches when the branch is deleted (delete event; GitHub only runs that event's workflows from the default branch, so release/10.x needs no copy). A daily and manual sweep also deletes every cache whose refs/heads/* branch is gone. That covers a deletion the event missed and clears the existing backlog once it has run.

The logic lives in .github/scripts/, like pr-hygiene.sh, so it can be run locally with --dry-run.