Clean stale temp directories

Builds occassionally leave tempdirs behind, probably when they failed/halted. The name prefixes of them are known to the app.

It would be good to have a check for them , warning they exist, and a 'purge' subcommand to delete them all.

Begin by tracing where builds create their tempdirs and where CLI subcommands are defined. Use the app's known name prefixes as the scope; done means stale directories are reported and a purge command removes all matching directories. The issue provides no test or file paths, so identify those before implementation.

Written by the indexing model from the issue text.