The base set (skill-creator + base@hyperhive) was declared via the
option's `default`, so a per-agent definition of claudePlugins
replaced it wholesale. Move the base set to a plain `config`
definition instead: a listOf option merges multiple plain definitions
by concatenation, so an agent's own list now adds to the base set
rather than replacing it, while lib.mkForce / lib.mkOverride on the
agent side still replace the whole merged list deliberately (mkDefault
was ruled out explicitly).
Also de-dup at the JSON-render site with lib.unique, so an agent that
names a base-set entry itself doesn't get it installed twice, and
reword the option doc, which still claimed the old REPLACES semantics.
Four module-eval cases cover the unset / agent-adds / mkForce-replaces
/ duplicate-entry shapes.
Refs #4467
b6e180dfbf