Add integrations/{emacs,systemd}: pm Emacs package and user units
Combines the previously separate extensions/emacs add, README path
fix, and extensions/systemd add into a single integrations/* layout —
mirroring emacs-config's integrations/ convention.
integrations/emacs/ ships the pm.el package (project.el backend,
magit-style transients, 21 ERT tests).
integrations/systemd/ ships pm-repo-mgmt.{service,timer} for keeping
the canonical repo store warm.
Wired into `make check` via the existing test-emacs target.