plugins_cmd.py had grown to 2,858 lines, past the ~2,000-line gate. Move
each verb family into a plugins_cmd_<topic>.py sibling: git (install
metadata + git plumbing), install, update (plus adopt / trust-update-url /
check-updates), remove, capabilities, toggle (composite UI) and listing.
The facade keeps the shared primitives, enable/disable selection,
discovery and the dispatch table, and re-exports the names other modules,
tests and the old-updater surface import (978 lines now).
Siblings never import the facade at module level; they read facade names
through _pc() at call time, so monkeypatching plugins_cmd.<name> still
intercepts calls made from a sibling. No behaviour change. Tests that
imported three sibling-only helpers now import them from the defining
module, and two subprocess.run patches target subprocess directly.
