chore: archive observability-service-registry, track pi-map artifacts

- Archive the completed observability-service-registry SDD change into
  openspec/changes/archive/ (delivered across 5 slices; only
  jellyfin-service-registry remains active).
- Stop ignoring .pi-map.md / .pi-map.index.md so the navigation maps are
  versioned alongside the code, and add the regenerated map pairs repo-wide.

monitoring/prometheus/.pi-map.md

@@ -0,0 +1,19 @@
+# monitoring/prometheus
+dir: monitoring/prometheus
+
+index: monitoring/prometheus/.pi-map.index.md
+
+## role
+Provides configuration for standalone Prometheus deployments to scrape and collect metrics from local infrastructure services and remote nodes.
+## files
+- prometheus.standalone.yml | Configures Prometheus monitoring server to scrape metrics from itself, node exporters, Loki, Alertmanager, Grafana, and remote nodes via file-based service discovery | dep: alertmanager, node-exporter, loki, grafana, file-sd
+## arch
+Uses a declarative static YAML configuration pattern leveraging file-based service discovery for dynamic remote target management.
+## tags
+node, loki, alertmanager, grafana, prometheus.standalone, configures, prometheus, monitoring
+## symbols
+-
+## workflows
+-
+## dirty
+-