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/alertmanager/.pi-map.index.md

@@ -0,0 +1,19 @@
+# monitoring/alertmanager (index)
+dir: monitoring/alertmanager
+
+## role
+Central configuration package for Prometheus alert routing, grouping, and notification delivery within the monitoring stack.
+## parent
+index: monitoring/.pi-map.index.md
+map: monitoring/.pi-map.md
+## children
+-
+## files
+- alertmanager.yml
+## links
+index: monitoring/alertmanager/.pi-map.index.md
+map: monitoring/alertmanager/.pi-map.md
+## workflows
+-
+## dirty
+-