agents: read the hive's name and domain at runtime, not from the build

Part of #5081 (slice S0-1a). An agent's closure no longer depends on the
name or domain of the hive that renders it.

- hive-priv writes /etc/hyperhive-hive-facts (HYPERHIVE_HIVE_NAME,
  HYPERHIVE_HIVE_DOMAIN) into the container from the same WriteNspawnFlags
  call that writes /etc/hyperhive-bridge-dns. hive-c0re passes the values
  from its own unit env. hive-priv refuses a value outside [A-Za-z0-9._-].
- A systemd environment generator in the agent (hive-facts.nix) loads the
  file into the container manager's environment, so every unit sees it,
  as with the systemd.globalEnvironment entries the render used to emit.
  It also builds CLAUDE_REMOTE_CONTROL_SESSION_NAME_PREFIX
  (<hive>-<agent>), which left claude's managed settings.
- meta.rs stops forwarding HYPERHIVE_HIVE_{NAME,DOMAIN} and stops setting
  services.hyperhive.agent.hiveName. The option is removed (both
  spellings now fail with a removal message).
- otel.nix drops the agent's hive= resource label. The swarm collector
  upserts hive per authenticated receiver (swarm-otel.nix resource/<h>,
  on both the metrics and logs pipelines), so stored series keep it.

The otel first hop, otel enable and the dashboard port are still baked;
they are S0-1b.

docs/process/conventions.md

@@ -38,6 +38,13 @@ Four env vars cover the identity surface, read by
   federated hives at different DNS domains can share a swarm
   name.
 
+Inside an agent container, the hive's name and domain don't come from the
+agent's build. `hive-priv` writes them to `/etc/hyperhive-hive-facts` before
+every container start, and the agent's environment generator
+(`nix/agent-modules/hive-facts.nix`) passes them to every unit. Neither is
+part of an agent's closure, and an agent started on another hive reports
+that hive.
+
 `hive_name` + `swarm_name` are **distinct** from
 `HYPERHIVE_HIVE_DOMAIN`: the domain may carry the hive name as
 its leftmost label by convention, but the convention isn't