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-c0re/src/meta.rs

@@ -325,6 +325,8 @@ const CANONICAL_INPUTS: &[&str] = &["nixpkgs", "nixpkgs-unstable"];
 /// touching process-wide env).
 const FORWARDED_VARS: &[&str] = &[
     "HIVE_FORGE_URL",
+    "HIVE_FORGE_PUBLIC_URL",
+    "HYPERHIVE_PEERS",
     "HYPERHIVE_HIVE_DOMAIN",
     "HYPERHIVE_HIVE_NAME",
     "HYPERHIVE_SWARM_NAME",