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.

CLAUDE.md

@@ -223,7 +223,7 @@ read them à la carte.
   bridge network for?"** → [`docs/network.md`](docs/network.md).
 - **"How do I connect two hives into a swarm? How do I declare peer
   hives and configure TLS trust?"** →
-  [`docs/swarm.md`](docs/swarm.md).
+  [`docs/swarm/`](docs/swarm/README.md).
 - **"Where do agent snapshots go? How does the swarm's `btrfs receive`
   endpoint authenticate a pushing hive, and what does a snapshot
   actually contain?"** →