vendor select <name> --add/--remove/--mode all changes what a source
contributes without advancing the pinned rev: ids are validated against
the pinned upstream (typos abort listing available ids), collisions
re-run, renames from the lockfile are honored, and the external area is
rewritten via the same staging-swap as update. include mode grows and
shrinks the include list; all mode --remove moves ids onto exclude;
--mode all resets to everything. Failed selects mutate nothing.
Closes beads: agent-lib-cd3
