HIVE_AGENT_NATS_URL now reaches each container in its hive facts, from
the host's deploy.hive-controller.queue.agentNatsUrl, instead of being
rendered into the meta flake. The agent option
services.hyperhive.agent.queue.natsUrl is removed; the harness unit
imports the queue credential whenever the token endpoint is set.
On deploy every agent rebuilds once (the rendered flake loses the
address), and the harness reads the address from the facts file
hive-priv writes before each start.
Refs #4857
hive-c0re's render_flake and the pieces it needs (AgentSpec, AgentPin,
pinned_git_url, the forwarded-var tables, canonical_inputs_in_lock) move
into a new crate with no IO. The forwarded values and the GitHub switch
are now arguments; hive-c0re passes what it read from its own unit env,
as before. Nothing changes on deploy: the rendered flake is byte-for-byte
the same, and hive-c0re's render tests pass unchanged.
Refs #4857
