feat(server): add Grafana dashboard and coverage test for all Prometheus metrics

- Ship examples/grafana-dashboard.json with panels for all 22 registered
  sortie_ metrics, including the four previously missing ones:
  sortie_dispatch_transitions_total, sortie_tracker_comments_total,
  sortie_ci_status_checks_total, and sortie_ci_escalations_total.
- CI metrics placed in a new "CI Feedback" row; dispatch and tracker
  comment counters placed in the existing "Integration" row.
- Dashboard uses __inputs / DS_PROMETHEUS pattern for portable import.
- Add grafana_coverage_test.go: TestGrafanaDashboardCoversAllMetrics parses
  the JSON and asserts every knownSortieMetrics entry appears in at least
  one panel PromQL expression, guarding against metric-to-panel drift.
- Extend metrics_test.go to exercise IncTrackerComments, SetSSHHostUsage,
  IncCIStatusChecks, and IncCIEscalations so the full metric list is
  covered by TestNewPromMetrics.

Author: serghei-dev