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/lib.rs

@@ -4,6 +4,7 @@
 pub mod client;
 pub mod events;
 pub mod forge_notify;
+pub mod identity;
 pub mod login;
 pub mod login_session;
 pub mod mcp;