fix(nix): swarm-controller is not a core binary, and it needed a README

Per review: daemonBins is the core stack, and it drives the bundle that
services.hyperhive.c0re.package points at -- so listing a swarm-scoped
service there would put it in every hive's closure when one hive in a
swarm runs it. It gets the same per-bin extractor, bound on its own, the
way hivectl already is.

The crate also had no README while every other one does. Both misses are
the same shape: adding a thing without updating what describes the set of
things.

swarm-controller/Cargo.toml

@@ -1,6 +1,7 @@
 [package]
 name = "swarm-controller"
 version.workspace = true
+readme = "README.md"
 edition.workspace = true
 
 [[bin]]