after setup_proposed + setup_applied, spawn now syncs the meta
flake (one input + one nixosConfiguration per agent) so
`--flake /var/lib/hyperhive/meta#<name>` resolves before
nixos-container create runs. flake ref switches from
applied/<n>#default to meta#<name>; the wrapper modules
(identity, HIVE_PORT, HIVE_LABEL, HIVE_DASHBOARD_PORT) now
live in the meta flake's mkAgent. new helper agents_for_meta
builds the AgentSpec list by enumerating containers + optionally
appending a not-yet-present name for the spawn case. spawn
keeps its caller signature; rebuild + auto_update get wired up
in follow-up commits.
8f94e4379a