hive-ag3nt: identity module with hive-qualified label (#589 phase A, first PR)

First chunk of #589 v0 phase A: plumbing the hive-qualified
'name@hive' form through the per-agent surfaces that the harness
itself owns. Broker from/to + dashboard rendering + container_view
follow in subsequent PRs once damocles ships the HYPERHIVE_HIVE_DOMAIN
env var in harness-base.nix.

- new hive_ag3nt::identity module: label() / hive_domain() /
  qualified_label() / qualify(label). Reads HYPERHIVE_HIVE_DOMAIN
  (set by hive-c0re.nix module from hyperhive.domain) — when unset
  or empty, qualified_label degrades to just the short label so
  existing single-hive deployments are unchanged. Six unit tests
  cover the set / unset / empty / arbitrary-label paths.
- prompt::render gains {qualified_label} substitution alongside
  the existing {label}. system.md template uses both: the agent
  intro now reads 'You are hyperhive agent iris (qualified:
  iris@darkest.space) in a multi-agent system. ... When you're
  talking to or about a peer on a different hive, use the
  qualified form (name@hive) so the operator + the manager can
  disambiguate'. Manager flavor gets the same treatment.
- /api/state gains qualified_label: String. Always present, equals
  label when no domain is configured.
- frontend setHeader takes the qualified_label, drives the browser
  tab title (so two  tabs from different hives are
  distinguishable in the tab bar) while the glyphic #title stays
  short for the cinematic header.

Gated on env var presence — no behaviour change for single-hive
deployments. Pairs with damocles's upcoming harness-base.nix
HYPERHIVE_HIVE_DOMAIN ship; safe to land in either order.

hive-ag3nt/src/web_ui.rs

@@ -340,6 +340,12 @@ struct StateSnapshot {
     /// harness restart — clients treat reconnect as a fresh world.
     seq: u64,
     label: String,
+    /// Hive-qualified long name (`${label}@${hyperhive.domain}`) when
+    /// the host has been configured for a multi-hive swarm; falls back
+    /// to the short label when the hive domain env var is unset (#589).
+    /// The frontend uses this for the page title / agent self-introduction;
+    /// when it equals `label`, the page renders the short form unchanged.
+    qualified_label: String,
     dashboard_port: u16,
     /// `"online"` | `"rate_limited"` | `"needs_login_idle"` | `"needs_login_in_progress"`.
     status: &'static str,
@@ -515,6 +521,7 @@ async fn api_state(State(state): State<AppState>) -> axum::Json<StateSnapshot> {
     axum::Json(StateSnapshot {
         seq,
         label: state.label.clone(),
+        qualified_label: crate::identity::qualify(&state.label),
         dashboard_port,
         status,
         session: session_view,