Add runtime dev release cleanup workflow

[d4rken]

Summary

• add a scheduled/manual cleanup workflow for immutable runtime-dev-YYYYMMDD-<sha> prereleases
• keep newest 5, anything 14 days old or newer, and the config-dev pinned runtime tag
• add a reusable pruning helper plus Bats coverage for retention and deletion behavior

Notes

• GitHub scheduled workflows are loaded from the default branch, so the daily 04:17 UTC cron will not start firing until this workflow file reaches the default branch. Push and manual-dispatch triggers work on dev immediately.

Tests

• bash -n runtime-overlay/scripts/release-workflow/prune-dev-runtime-releases.sh
• shellcheck runtime-overlay/scripts/release-workflow/prune-dev-runtime-releases.sh
• bats test/runtime-overlay/test_prune_dev_runtime_releases.bats
• git diff --check