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.index.md

@@ -0,0 +1,24 @@
+# monitoring/prometheus (index)
+dir: monitoring/prometheus
+
+## role
+Provides configuration for standalone Prometheus deployments to scrape and collect metrics from local infrastructure services and remote nodes.
+## parent
+index: monitoring/.pi-map.index.md
+map: monitoring/.pi-map.md
+## children
+- monitoring/prometheus/file-sd
+  index: monitoring/prometheus/file-sd/.pi-map.index.md
+  map: monitoring/prometheus/file-sd/.pi-map.md
+- monitoring/prometheus/rules
+  index: monitoring/prometheus/rules/.pi-map.index.md
+  map: monitoring/prometheus/rules/.pi-map.md
+## files
+- prometheus.standalone.yml
+## links
+index: monitoring/prometheus/.pi-map.index.md
+map: monitoring/prometheus/.pi-map.md
+## workflows
+-
+## dirty
+-