docs: split swarm.md into a directory, starting with the services page

`docs/swarm.md` becomes `docs/swarm/README.md` and the shared-services
material moves to `docs/swarm/services.md`, following the shape
`docs/turn-loop/` and `docs/web-ui/` already use. The README keeps a
pointer so the reading path is unbroken.

Every referrer moved with it — five docs pages, two option descriptions
in swarm.nix, and CLAUDE.md's reading path. A pointer to a file that
moved is worse than one to a file that was deleted: the content still
exists, so the reader concludes the note is wrong rather than the path.

docs/network.md

@@ -66,7 +66,7 @@ The flows, end to end:
   the host forwards + masquerades out its uplink.
 - **Swarm** — peer hives connect over the `wg-hive` WireGuard mesh
   and reach each other's gateway/forge across it
-  ([`docs/swarm.md`](swarm.md)).
+  ([`docs/swarm/`](swarm/README.md)).
 - **Control plane (no network)** — per-agent broker socket
   `/run/hive/mcp.sock`, privileged helper `/run/hive/priv.sock`,
   operator admin `/run/hyperhive/host.sock`, and the per-agent UI