Add a demo host-metrics fixture

Pushes synthetic cairnobs.metrics/cairnobs.heartbeat-tagged records
straight to ingest so a demo deployment's Hosts page and per-host detail
page have CPU/memory/disk to show. Neither benchmark-fixture nor
windows-fixture emits those two attributes -- only the real agent does
-- so nothing else could populate that page.

Superseded by /hack/demo-simulator in the next commit, which covers the
same records as part of a whole fleet; kept because it is still the
smallest thing that fills the Hosts page on its own.

hack/demo-hosts-fixture/go.mod

@@ -0,0 +1,18 @@
+module github.com/cairnobs/cairnobs/hack/demo-hosts-fixture
+
+go 1.25.0
+
+replace github.com/cairnobs/cairnobs/proto => ../../proto
+
+require (
+	github.com/cairnobs/cairnobs/proto v0.0.0-00010101000000-000000000000
+	google.golang.org/grpc v1.83.0
+)
+
+require (
+	golang.org/x/net v0.55.0 // indirect
+	golang.org/x/sys v0.45.0 // indirect
+	golang.org/x/text v0.39.0 // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20260526163538-3dc84a4a5aaa // indirect
+	google.golang.org/protobuf v1.36.12 // indirect
+)