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/mod.rs

@@ -177,6 +177,17 @@ pub fn network_isolation_from_env() -> Result<hive_priv_sock::NetworkIsolation>
     network_isolation_from_vars(bridge.as_deref(), subnet.as_deref())
 }
 
+/// The hive's facts every container is handed at runtime, from hive-c0re's
+/// own unit environment (`nix/host-modules/hive-c0re/environment.nix`). An
+/// unset or empty variable is `None`.
+fn hive_facts_from_env() -> hive_priv_sock::HiveFacts {
+    let read = |name: &str| std::env::var(name).ok().filter(|v| !v.is_empty());
+    hive_priv_sock::HiveFacts {
+        name: read("HYPERHIVE_HIVE_NAME"),
+        domain: read("HYPERHIVE_HIVE_DOMAIN"),
+    }
+}
+
 fn validate(name: &str) -> Result<()> {
     if name.is_empty() {
         bail!("agent name must not be empty");