feat(nix): move the forge host options under services.hyperhive.swarm

Forgejo is a swarm-global service, so its operator-facing host options
move to services.hyperhive.swarm.forge (and .swarm.forge.ci) as the
first of the namespace consolidation.

Existing hive configs keep evaluating: swarm-renames.nix maps every
moved leaf with mkRenamedOptionModule, which also emits a deprecation
warning naming both the old and new path, so an operator is told what to
rename rather than discovering it from a failed eval.

The per-agent hyperhive.forge.url does NOT move. It is a client pointer
at whatever forge an agent talks to - it shares a word with the service
and nothing else, and the two are already documented as separate option
surfaces.

Verified by evaluating the host module, since no Rust gate evaluates
nix: setting the old paths and reading the new ones yields the values
(httpPort 3999, ci.concurrency 7), and config.warnings carries the
rename notice.

nix/host-modules/default.nix

@@ -12,6 +12,7 @@
 {
   imports = [
     ./hyperhive.nix
+    ./swarm-renames.nix
     ./hive-c0re
     ./hive-ci.nix
     ./hive-forge