feat(infra): remove Go dashboard, dashboard is now part of hub Next.js app

- Remove dashboard container from observability.yml
- Remove infra/docker/dashboard.Dockerfile
- Remove dashboard route from Traefik apps.yaml
- Mount Docker socket on hub container for /api/dashboard
- Remove dashboard build from CI/CD (docker-build-push.yml)
- Update submodule pointer to hub with Next.js dashboard rewrite

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

infra/docker/dashboard.Dockerfile

@@ -1,11 +0,0 @@
-# ── Build stage ──
-FROM golang:1.24-alpine AS builder
-WORKDIR /build
-COPY apps/hub/dashboard/ ./
-RUN CGO_ENABLED=0 go build -ldflags="-s -w" -o dashboard .
-
-# ── Runtime (scratch — ~7MB total) ──
-FROM scratch
-COPY --from=builder /build/dashboard /dashboard
-EXPOSE 8080
-CMD ["/dashboard"]