The default `http://localhost:3000` was a guess, and a guess that
evaluates is worse than one that doesn't: the forge may run on a
different host from the agents, and inside an agent's network namespace
`localhost` reaches the agent itself, not the forge. hive-c0re renders
this option into every agent's config from the host's `HIVE_FORGE_URL`,
which `hive-c0re.nix` sets unconditionally, so nothing legitimate was
relying on the default to be there.
Also tightens the URL assertion, which accepted `""`. That escape hatch
only existed because the default made the empty case unreachable; with
the default gone, `""` is exactly what a caller supplies when they have
nothing, so it now fails naming the option instead of silently building
a config that talks to nowhere.
Refs #2860
7ef9905e9f