docs+release: Phase 4 launch artifacts

README hero with real screenshots, install/config/widget docs, goreleaser
for dash + dash-agent, GHCR release workflow, changelog, community files,
agent systemd unit and Dockerfile, version stamping via -X main.version.

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infra/Dockerfile.agent

@@ -0,0 +1,15 @@
+# syntax=docker/dockerfile:1
+
+# Minimal dash-agent image. A containerized agent only sees its own /proc —
+# run with --pid=host and -v /proc:/host/proc for host metrics, or prefer the
+# systemd unit in packaging/.
+FROM golang:1.27-alpine AS build
+WORKDIR /src
+COPY apps/backend/go.mod apps/backend/go.sum ./
+RUN go mod download
+COPY apps/backend ./
+RUN CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o /out/dash-agent ./cmd/dash-agent
+
+FROM gcr.io/distroless/static-debian12
+COPY --from=build /out/dash-agent /dash-agent
+ENTRYPOINT ["/dash-agent"]