A new update_meta node renders internal/meta's flake.nix with
hive-meta-flake, the renderer hive-c0re uses: hyperhive pinned to its
forge main, each custom agent a hive places pinned to its agent-configs
main, all over https, and the swarm-wide values from
SWARM_CONTROLLER_META_FLAKE (set by swarm-controller.nix from
services.hyperhive.swarm.*). The swarm builder locks it, and the
controller commits flake.nix and flake.lock in one commit and gives
every hive's forge account read on the repo. A render equal to the
committed flake.nix sends no lock and commits nothing.
It runs at start, when hyperhive's main moves, before the deploy of a
merged config PR and before the first deploy of a new agent; those
deploys now wait for it. agent_web_port moves into hive-meta-flake so
both callers compute the same port; hive-c0re re-exports it.
On deploy: internal/meta appears on the forge and follows every merge.
No hive builds from it yet.
Refs #4857
