postprocess: add --gc-cache to garbage collect any cache entries not used since the last --gc-cache call

[kkysen]

When there are input changes (like transpiler, refactorer, prompt, etc. changes), the cache becomes outdated and must be recalculated, but the previous entries aren't deleted. This tries to solve that. It tracks which cache entries are still actively tested/used (this is always done in llm-cache/.gc), and then --gc-cache deletes everything else. So the normal intended usage is to:

• Run rm -f llm-cache/.gc.
• Run all tests, updating the cache with new entries.
• Run with --gc-cache to remove the outdated, unused entries.

I tried to do this in a simple way, but it does seem pretty necessary once testing is added to CI and others will need to do the same on their own, instead of me manually deleting the right outdated cache entries. If there are better/simpler ways to do this, that would also be great.

[kkysen]

I've detached --gc-cache from CommentTransferOptions now (it was never really needed there) and switched to an mtime-based version, but I haven't gotten around to fully separating --gc-cache into a separate script yet, which requires some more work, as part of the script will have to be deduplicated/set up to be imported.

[kkysen]

This is now independent of #1553 and rebased on master, so it can merge independently.

Also, the mtime and ctime-based implementation was broken because apparently on Linux, ctime is change time, not creation time, so I've switched to a purely mtime-based implementation now and tested it.

[kkysen]

If we want to separate this into a separate script, we'll have to separate out this cache.gc_sweep() call, the cache creation (cache = getattr(DirectoryCache, args.cache_scope)()), and thus args.cache_scope as well. So we'd have to duplicate a bunch of stuff here, or refactor things such that we can import them and only them (like only --cache-scope, not the other arguments from build_arg_parser).