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.

frontend/src/widgets/.pi-map.index.md

@@ -0,0 +1,27 @@
+# frontend/src/widgets (index)
+dir: frontend/src/widgets
+
+## role
+Provides a collection of self-contained, UI-driven dashboard widgets that fetch and render data from various external services and integrations.
+## parent
+index: frontend/src/.pi-map.index.md
+map: frontend/src/.pi-map.md
+## children
+-
+## files
+- AlertmanagerAlertsWidget.tsx
+- BackupsWidget.tsx
+- GrafanaLinkWidget.tsx
+- JellyfinWidget.tsx
+- PrometheusMetricWidget.tsx
+- SshTaskWidget.tsx
+- StaticWidget.tsx
+- index.ts
+## links
+index: frontend/src/widgets/.pi-map.index.md
+map: frontend/src/widgets/.pi-map.md
+## workflows
+- change widgets behavior
+  read: AlertmanagerAlertsWidget.tsx, BackupsWidget.tsx, GrafanaLinkWidget.tsx
+## dirty
+-