feat(#589): swarm peers option + HYPERHIVE_PEERS env wire v0

nix: services.hyperhive.peers attrset-of-submodules option; serialises
to HYPERHIVE_PEERS JSON ([{label,domain}]); forwarded to containers via
FORWARDED_VARS. dashboard.rs: peer_hives: Vec<PeerHiveView> in
StateSnapshot, derived as {name:label, url:"http://domain/"}.
identity.rs: PeerHive struct + peers() accessor for agent-side use.

hive-ag3nt/src/identity.rs

@@ -48,6 +48,25 @@ pub fn swarm_name() -> Option<String> {
         .filter(|s| !s.is_empty())
 }
 
+/// One peer hive in the same swarm. Parsed from `HYPERHIVE_PEERS`.
+#[derive(Debug, Clone, serde::Deserialize)]
+pub struct PeerHive {
+    pub label: String,
+    pub domain: String,
+}
+
+/// Peer hives in the same swarm, parsed from `HYPERHIVE_PEERS` env var
+/// (JSON array of `{label,domain}` objects, emitted by the c0re NixOS
+/// module from `services.hyperhive.peers`). Returns empty vec on
+/// single-hive deploys (env var absent).
+#[must_use]
+pub fn peers() -> Vec<PeerHive> {
+    env::var("HYPERHIVE_PEERS")
+        .ok()
+        .and_then(|s| serde_json::from_str(&s).ok())
+        .unwrap_or_default()
+}
+
 /// Hive-qualified agent identity. When the hive domain is configured, returns
 /// `${label}@${domain}` (e.g. `iris@darkest.space`); when not, returns just
 /// the short label so callers can render a single string regardless of