fix(nix): the rename lives with the module that declares the options

mara: "remove the extra deprecatios file, less comments". The alias now
sits in hive-forge/default.nix next to the options it renames, and the
comment is three lines instead of eighteen.

Better than tidier: there is no shared file for the sibling migrations
to queue behind. Matrix's rename goes in hive-matrix.nix, authelia's in
its own module, and none of them touch each other. hive-ci.nix declares
swarm.forge.ci.* from a different module and stays covered, because the
rename is on the namespace rather than on the declaring file.

Re-ran the check both ways this time, per argus on the last round: old
paths set and new paths read back 3999 / 7, and a config that only uses
the new paths produces no warnings at all.

nix/host-modules/hive-forge/default.nix

@@ -82,6 +82,16 @@ in
   # (base URL), the same shape as the GitHub PAT / matrix extra-account
   # flows. See `hive-c0re/src/dashboard/extra_forges.rs`.
 
+  # Forge moved under `swarm` when the swarm-global services were
+  # consolidated. One rename for the namespace: the subtree comes with it,
+  # so existing hives keep evaluating and get one warning naming both paths.
+  imports = [
+    (lib.mkRenamedOptionModule
+      [ "services" "hyperhive" "forge" ]
+      [ "services" "hyperhive" "swarm" "forge" ]
+    )
+  ];
+
   # The internal forge is mandatory — it's the canonical store for the
   # meta flake + every agent's config repo (and the `internal/*` repos),
   # so there is no enable/disable toggle. It deploys whenever hyperhive