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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packaging/dash-agent.service

@@ -0,0 +1,18 @@
+[Unit]
+Description=Dash system monitoring agent
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+EnvironmentFile=/etc/dash/agent.env
+ExecStart=/usr/local/bin/dash-agent
+Restart=always
+RestartSec=5
+
+# Reads /proc, /sys, and (if present) the Docker socket.
+ProtectHome=true
+NoNewPrivileges=true
+
+[Install]
+WantedBy=multi-user.target