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/pages/.pi-map.index.md

@@ -0,0 +1,34 @@
+# frontend/src/pages (index)
+dir: frontend/src/pages
+
+## role
+Top-level page components constituting the primary routed views of the frontend application, each encapsulating a distinct feature domain (dashboard, media, services, actions, files, settings, users).
+## parent
+index: frontend/src/.pi-map.index.md
+map: frontend/src/.pi-map.md
+## children
+- frontend/src/pages/__tests__
+  index: frontend/src/pages/__tests__/.pi-map.index.md
+  map: frontend/src/pages/__tests__/.pi-map.md
+## files
+- Actions.tsx
+- Applications.tsx
+- Dashboard.tsx
+- FileBrowser.impl.tsx
+- FileBrowser.tsx
+- Media.tsx
+- ServicePage.tsx
+- ServicesPage.tsx
+- Settings.tsx
+- Users.tsx
+- UsersPage.impl.tsx
+## links
+index: frontend/src/pages/.pi-map.index.md
+map: frontend/src/pages/.pi-map.md
+## workflows
+- change pages behavior
+  read: Actions.tsx, Applications.tsx, Dashboard.tsx
+- explore pages subdirectories
+  index: frontend/src/pages/__tests__/.pi-map.index.md
+## dirty
+-