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.

build/dockerfiles/Dockerfile

@@ -0,0 +1,21 @@
+# syntax=docker/dockerfile:1
+
+ARG GO_VERSION=1.25
+
+FROM golang:${GO_VERSION} AS build
+WORKDIR /src
+
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+
+ARG CMD
+RUN test -n "$CMD"
+RUN CGO_ENABLED=0 GOOS=linux GOARCH=$TARGETARCH go build -o /out/app ./cmd/$CMD
+
+FROM gcr.io/distroless/static:nonroot
+WORKDIR /
+COPY --from=build /out/app /app
+USER nonroot:nonroot
+ENTRYPOINT ["/app"]