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/README-build.md

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+Build/push images
+
+- Default builds linux/amd64 + linux/arm64 and pushes.
+- Default REGISTRY is `gitea-http.taildb3494.ts.net/will/agentmon` (your Gitea registry).
+
+Examples:
+
+- Push to default image names:
+  TAG=dev ./build/build-images.sh
+
+- Push to your registry:
+  REGISTRY=registry.docker-registry.svc.cluster.local:5000 TAG=dev ./build/build-images.sh
+
+- Build locally without pushing (single-arch only; Docker can’t `--load` multi-arch):
+  PLATFORM=linux/amd64 PUSH_ARGS=--load TAG=dev ./build/build-images.sh

build/build-images.sh

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+#!/usr/bin/env bash
+set -euo pipefail
+
+# Builds multi-arch images by default.
+#
+# Defaults:
+# - PLATFORM="linux/amd64,linux/arm64"
+# - PUSH_ARGS="--push" (set to --load for a single-arch local build)
+
+REGISTRY=${REGISTRY:-"gitea-http.taildb3494.ts.net/will/agentmon"}
+TAG=${TAG:-"dev-20260117-0832"}
+PLATFORM=${PLATFORM:-"linux/amd64,linux/arm64"}
+
+services=(
+  ingest-gateway
+  event-processor
+  query-api
+  web-ui
+)
+
+for svc in "${services[@]}"; do
+  image="${REGISTRY}/${svc}:${TAG}"
+
+  echo "==> Building ${image} (${PLATFORM})"
+  docker buildx build \
+    --platform "${PLATFORM}" \
+    --build-arg CMD="${svc}" \
+    -f build/dockerfiles/Dockerfile \
+    -t "${image}" \
+    ${PUSH_ARGS:-"--push"} \
+    .
+done

build/dockerfiles/Dockerfile

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+# 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"]