Add tool to clean up old tasks not in index

Previously the `drop` command did not remove the symlink in .tsk/tasks when a
task was dropped, only removing it from the index. I don't recall if this was
because my original design expected the index to be the source of truth on
whether a task was prioritized or not or if I simply forgot to add it. Either
way, drop now properly removes the symlink but basically every existing
workspace has a bunch of junk in it. I can write a simple script that deletes
all symlinks that aren't present in the index.

This does suggest that I should add a reindex command to add tasks that are not
present in the index but are in the tasks folder to the index at the *bottom*,
preserving the existing order of the index.


How about just adding a `fixup` command that does this and reindex as laid out in
[[tsk-18]].