chore: remove unused desktop app, cloud deployment infra, and monitoring stack

Deletes desktop/ (Electron wrapper), deployments/ (Helm/Kustomize/
Terraform/CI for cloud deploy), and monitoring/ (Prometheus + Grafana
stack) along with DESKTOP.md, DEPLOYMENT.md, docker-compose.full.yml,
their npm scripts, and every dangling reference across README,
ARCHITECTURE, INSTALL, SETUP, docs/, and the repeated per-file
MODULE_GUIDE "Observability" boilerplate comment. The GET /api/metrics
endpoint itself is untouched — it's the dashboard's own route, not
part of the removed monitoring stack.

docker-compose.yml

@@ -15,8 +15,8 @@ services:
     environment:
       - NODE_ENV=production
       - DASHBOARD_PORT=4820
-      # Optional — required when Prometheus-in-Docker scrapes this container
-      # (host.docker.internal) or the full stack (agent-monitor). See monitoring/README.md.
+      # Optional — required when a Prometheus-in-Docker scraper reaches this
+      # container as anything other than loopback (e.g. host.docker.internal).
       - DASHBOARD_ALLOWED_HOSTS=${DASHBOARD_ALLOWED_HOSTS:-}
       # DASHBOARD_HOST=0.0.0.0 and DASHBOARD_DATA_DIR=/app/data are baked into
       # the image (see Dockerfile) so both `docker run` and Compose work as-is.