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.

hive-c0re/src/lifecycle/host_config.rs

@@ -476,7 +476,14 @@ async fn set_nspawn_flags(
     );
 
     // Delegate the actual conf-file rewrite to hive-priv (runs as root).
-    crate::priv_client::write_nspawn_flags(container, &binds, isolation, &load_creds).await
+    crate::priv_client::write_nspawn_flags(
+        container,
+        &binds,
+        isolation,
+        &load_creds,
+        super::hive_facts_from_env(),
+    )
+    .await
 }
 
 #[cfg(test)]