docs(conventions): fix one more stale hive-c0re.nix bare-file ref

Same bug class the facts sweep hunted for across everything else —
hive-c0re.nix is a module directory now
(nix/host-modules/hive-c0re/), environment.nix is where
HYPERHIVE_HIVE_DOMAIN is actually set. Caught doing a final grep
across the merged sweep for the known bad patterns before pushing.

docs/conventions.md

@@ -24,8 +24,8 @@ Four env vars cover the identity surface, read by
   surface "unknown agent" rather than getting a panic from this
   module.
 - `HYPERHIVE_HIVE_DOMAIN` — the hive's canonical DNS domain (e.g.
-  `darkest.space`), set by `hive-c0re.nix` from
-  `services.hyperhive.domain`. When configured, `qualified_label()`
+  `darkest.space`), set by `nix/host-modules/hive-c0re/environment.nix`
+  from `services.hyperhive.domain`. When configured, `qualified_label()`
   returns `${label}@${domain}` (e.g. `iris@darkest.space`); when
   unset (single-hive deployments, dev/test) it degrades to just
   the short label so existing callers see no change. The