Review feedback: the metrics pair had nothing writing into it, and it sat
outside the switch that turns on every other swarm-wide service.
The collector now exports to VictoriaMetrics as well as upstream — a
fan-out, not a choice: a local store is for looking at this swarm, an
upstream is for whoever aggregates across swarms.
That makes a local store a complete destination on its own, so
`otel.endpoint` is no longer required when it runs here; a hive with
neither is still refused. The assertion only ever relaxes, so every
config that evaluated before still does.
`enableRequiredServices` now derives both halves, alongside matrix,
authelia and nats. They derive together because a store with no UI is
unreadable and a UI with no store is empty.
`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.
