The option had a `http://localhost:3000` default, which is only ever
correct when the forge shares the caller's network namespace — inside an
agent's netns `localhost` is the agent, and the forge may well be on
another host. Making it *required* instead was worse: the flake's own
container configs are what hive-c0re extends per agent, so the value
they needed in order to evaluate became a second definition on every
agent and collided with the real one.
`null` resolves both. It is not a URL, so nothing can quietly talk to
the wrong machine, and it needs no placeholder anywhere: the bases
evaluate as they are, so nothing deployment-shaped sits on the config
agents inherit from. The units that would consume the URL — tea-login
and forge-avatar-sync — are simply not generated without one, making an
absent forge an absent integration rather than a misdirected one.
hive-forge-notify is unaffected: it reads HIVE_FORGE_URL from the
forwarded global environment, not from this option.
Verified:
agent-base/ruth evaluate with forge.url = null, zero failing assertions
bare base: tea_login_present = false, avatar_present = false,
notify_present = true
extended with a rendered URL: FORGE_URL=http://forge.real.test
Refs #2860
bcb9e837f7