Add Grafana lint gate and refresh DCUtR dashboard

[MontrealAI]

Summary

• Rebuild the DCUtR Grafana dashboard stub with the requested UID, title, and panel PromQL targets and document the updated panel guide
• Add a Grafana lint script with Docker/local CLI fallback, wire it into CI and required checks, and surface a dashboard lint badge
• Refresh the README to highlight the new observability primer, Grafana linting, and enforcement details for the CI wall

Testing

• npm run lint:grafana
• npm test -- test/observability/metrics_dcutr.test.ts
• npm run test
• npm run test:frontend

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Codex Task

[chatgpt-codex-connector]

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