Rework cleanup scripts and periodic execution

[kergoth]

I have various scripts for periodic cleanup operations:

• brew-clean
• nix-clean
• clean-sstate (makes assumptions about shared state paths)
• safari-webapp-cache-clean
• brew-clean-cask-pkgs

And other potential ones, like docker system prune and the like. Consolidate them into a wrapper structure of some sort, and integrate that with periodic cleanup operation services and launchd agents.

I'm leaning toward a clean or cleanup wrapper script which looks for scripts in a clean.d or cleanup.d folder by the script, but also git-like looks for clean-* or cleanup-* subscripts in the PATH to run. We can add commented header metadata to script files to use in service generation, if they have differing scheduling preferences, if worthwhile.

[kergoth]

I'm unsure if this is worth fitting into a wrapper structure, but at the least I think it's worth aligning them on a clean- file prefix for consistent naming.