Automatically remove cached engine builds

Expert stores engine builds in a shared user cache. Over time, cached builds for old Elixir/OTP combinations can accumulate and grow disk usage.

We already provide manual cleanup with `expert engine clean`, but many users are unlikely to know about it or run it regularly. It would be better if the cache cleaned up stale builds automatically.

Proposal:
- track when each cached engine build was last used
- automatically remove builds that have not been used for 30 days
- run cleanup opportunistically and as a best-effort operation, so it does not block normal startup or engine use
- doing this as a post-step of engine build looks could be work well

This should keep the cache bounded without affecting active projects. Rebuilding an engine after 30 days of inactivity seems like a reasonable tradeoff.

_Originally posted by @mkaput in [#466](https://github.com/elixir-lang/expert/issues/466#issuecomment-3972157086)_