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/docs/default.nix

@@ -153,7 +153,8 @@ let
 
     - [host options](host.md) — options exposed by
       `hyperhive.nixosModules.default` to operator host configurations
-      (`services.hyperhive.{enable,domain,c0re,forge,matrix,gateway}.*`).
+      (`services.hyperhive.{enable,domain,c0re,gateway}.*`,
+      `services.hyperhive.swarm.{forge,matrix}.*`).
     - [per-agent options](agent.md) — options declared in
       `nix/agent-modules/`, visible from every `agent.nix`
       (`hyperhive.model`, `hyperhive.allowedRecipients`,