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.

openspec/specs/.pi-map.index.md

@@ -0,0 +1,20 @@
+# openspec/specs (index)
+dir: openspec/specs
+
+## role
+Directory for OpenAPI/Swagger specification files defining API contracts, schemas, and endpoint documentation.
+## parent
+index: openspec/.pi-map.index.md
+map: openspec/.pi-map.md
+## children
+- openspec/specs/web-ui
+  index: openspec/specs/web-ui/.pi-map.index.md
+  map: openspec/specs/web-ui/.pi-map.md
+## files
+## links
+index: openspec/specs/.pi-map.index.md
+map: openspec/specs/.pi-map.md
+## workflows
+-
+## dirty
+-