swarm-builder: give the check flake the hive's docs source

The builder's hyperhive input is the docs-stripped flake source, so an
agent config with docs.enable = true failed to evaluate: docs.source
defaults to reference-docs, built from the missing docs/. Pass the host's
c0re.hyperhiveDocs as a hyperhive-docs input and set docs.source from it,
as the hive's meta flake does.

Fixes #5411

docs/swarm/builder.md

@@ -26,6 +26,10 @@ Where it runs and how it's enabled: [`services.md`](services.md#builder).
    - `hyperhive` and `nixpkgs`: the sources the builder's own host was
      deployed with, the same two a hive on that host renders into its meta
      flake (`c0re.hyperhiveFlake`, `c0re.nixpkgsFlake`).
+   - `hyperhive-docs`: the `docs/` source a hive on that host renders into its
+     meta flake (`c0re.hyperhiveDocs`). The check sets
+     `services.hyperhive.agent.docs.source` from it, as the meta flake does,
+     since the `hyperhive` source has no `docs/`.
    - `agent`: `git+https://<forge>/agent-configs/<agent>?ref=refs/pull/<pr>/head&rev=<head>`.
      A build has one hour.
 4. The reply is the system closure's store path, or the last lines `nix`
@@ -48,7 +52,7 @@ Where it runs and how it's enabled: [`services.md`](services.md#builder).
 
 - It composes the config the way a hive's meta flake does
   (`agent-base.extendModules` with the config's `nixosModules.default`), minus
-  what a hive adds: its CA bundle, docs source, `claude-code` path, ports and
+  what a hive adds: its CA bundle, `claude-code` path, ports and
   OTEL settings, and the `follows` that point the config's own inputs at the
   hive's. A green check can therefore still fail on a hive, and the reverse.
 - The agent base is the builder's host's, not the hive's. Hives on another