Add temporaryDirectory, alongside a way to remove a directory and its contents

This issue was triaged from bugs/dan/0.5-temporaryDirectory, one of the 857 files removed from the pre-GitHub bugs/ tree by d2c8d27826 and catalogued in #36. The commentary below was written by Claude (Claude Opus 5, via Claude Code), not by @d-torrance, whose account posted it -- please weigh it accordingly.

The original file, verbatim

Provide this function to the user:

    	temporaryDirectory = () -> while true do try (dn := temporaryFileName(); makeDirectory dn; return dn)

and also a function for removing it and its contents

Where it stands today

Not provided. temporaryDirectory does not exist, though removeDirectory does — so of the pair the
file asks for, only the removal half is available, and it removes an empty directory rather than a
directory and its contents.

The file supplies the implementation

temporaryDirectory = () -> while true do try (dn := temporaryFileName(); makeDirectory dn; return dn)

The retry loop is the point: temporaryFileName can collide, and makeDirectory on an existing path
fails, so the loop is what makes the result safe to use.

Why both halves are wanted

A package that needs scratch space today either invents its own loop or writes into a name from
temporaryFileName without creating a directory, and cleaning up afterwards requires walking the
contents by hand. Both are the sort of thing that should exist once in Core rather than once per package.

Notes for whoever picks this up

The removal half wants care: a recursive delete taking a directory name is a foot-gun, so it is worth
restricting it to directories this function created, or requiring the path to lie under the temporary
directory root.

_{open · disposition issue · source of truth: bug-triage/catalog.tsv}

Start by locating the existing temporaryFileName, makeDirectory, and removeDirectory entry points in Core and review how temporary paths are handled. Define the safe behavior for creating a unique temporary directory and removing its contents, then verify both operations and their failure cases with the project's relevant tests.