Rewrite ui-plan as a pure browser surface plugin. The control occupies the
composer's named plan seat (declared empty by ui-conversation); reads render
the host-computed plan projection through the standard-kit useProjection
(absent key = capability absence, hides the control), writes execute /plan
or /plan off through command.execute. The node half becomes the empty
roster apply: plan behavior (command, policy, projection unit) is owned by
dsh-plan-mode, already composed on the web roster with its policy in
cordis.yml. The superseded RPC-backed setPlanMode face, the WEB_PLAN_SECTION
duplicate, and the node-plugin spec are removed; the roster row moves from
the retired CLIENT_PACKAGES table to the cordis.yml dshClient roster.
