monopam: drop git-subtree-dir trailers, surface untracked subdirs
The trailer was only written, never read back; its sole use was
compatibility with stock git-subtree, which nothing in this workspace
relies on. It mostly confused anyone reading import/merge messages.
monopam status now reports an 'Untracked:' block listing monorepo
subdirs that contain a dune-project but aren't covered by any known
package.