emacs: set default-directory on pm buffers
Project-scoped buffers (pm-status, pm-agent-list when given a
project) bind default-directory to the container so transient,
project-current, and pm--container-of resolve to the right project
from inside the buffer.
Global buffers (pm-list, pm-pool, pm-repos, all-projects
pm-agent-list) bind default-directory to a new pm-manager-project
defcustom (a project name); when unset, they fall back to ~.