The connectivity-overrides section still documented
`hyperhive.forge.url = "http://localhost:3000"` as the default and said
the empty string was a valid value. Both stopped being true when the
option became `nullOr str` with `default = null`.
Also says what `null` means and why, since "the default is null" alone
reads like an oversight rather than the point: a loopback default is
only correct when the forge shares the agent's netns, and the units that
would consume the URL are simply not generated without one.
Refs #2860
Agents had the official marketplace configured out of the box but an
empty plugin list, so nothing was installed from it unless an agent's
own config asked. skill-creator is the one plugin that pays for itself
generically: it teaches an agent to write, refine, and evaluate its own
skills, which is exactly the capability an agent can't bootstrap by
being told about it once in a prompt.
Defaulting the option keeps this consistent with claudeMarketplaces,
which already ships the official marketplace the same way. Documented
the list-option semantics next to both: a per-agent definition replaces
the default rather than extending it, so an agent that sets its own
plugin list has to name skill-creator alongside its entries.
