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.

.gitignore

@@ -55,5 +55,3 @@ frontend/dist/
 .superpowers/
 # Local Pi runtime state
 .atl/
-.pi-map.md
-.pi-map.index.md