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/file-sd/.pi-map.index.md

@@ -0,0 +1,19 @@
+# monitoring/prometheus/file-sd (index)
+dir: monitoring/prometheus/file-sd
+
+## role
+Provides static target configuration for Prometheus to discover and scrape node_exporter metrics endpoints.
+## parent
+index: monitoring/prometheus/.pi-map.index.md
+map: monitoring/prometheus/.pi-map.md
+## children
+-
+## files
+- node_exporter_targets.json
+## links
+index: monitoring/prometheus/file-sd/.pi-map.index.md
+map: monitoring/prometheus/file-sd/.pi-map.md
+## workflows
+-
+## dirty
+-

monitoring/prometheus/file-sd/.pi-map.md

@@ -0,0 +1,19 @@
+# monitoring/prometheus/file-sd
+dir: monitoring/prometheus/file-sd
+
+index: monitoring/prometheus/file-sd/.pi-map.index.md
+
+## role
+Provides static target configuration for Prometheus to discover and scrape node_exporter metrics endpoints.
+## files
+- node_exporter_targets.json | Empty JSON file intended to define scrape targets for Prometheus node_exporter | dep: Prometheus, node_exporter
+## arch
+Uses the Prometheus file-based service discovery (file_sd) pattern with JSON-formatted target definitions.
+## tags
+targets, node, exporter, prometheus, empty, json, intended, define
+## symbols
+-
+## workflows
+-
+## dirty
+-