feat: scaffold agentmon services and k8s deploy

Adds Go microservices (ingest-gateway, event-processor, query-api, web-ui), NATS+Postgres wiring, initial schema/init job, ingress manifests for LAN+tailnet, and a multi-arch image build script.

go.mod

@@ -0,0 +1,23 @@
+module agentmon
+
+go 1.25.0
+
+require (
+	github.com/go-chi/chi/v5 v5.2.4
+	github.com/gorilla/websocket v1.5.3
+	github.com/jackc/pgx/v5 v5.8.0
+	github.com/nats-io/nats.go v1.48.0
+)
+
+require (
+	github.com/jackc/pgpassfile v1.0.0 // indirect
+	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
+	github.com/jackc/puddle/v2 v2.2.2 // indirect
+	github.com/klauspost/compress v1.18.0 // indirect
+	github.com/nats-io/nkeys v0.4.11 // indirect
+	github.com/nats-io/nuid v1.0.1 // indirect
+	golang.org/x/crypto v0.37.0 // indirect
+	golang.org/x/sync v0.17.0 // indirect
+	golang.org/x/sys v0.32.0 // indirect
+	golang.org/x/text v0.29.0 // indirect
+)