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.md

@@ -0,0 +1,18 @@
+# openspec/specs
+dir: openspec/specs
+
+index: openspec/specs/.pi-map.index.md
+
+## role
+Directory for OpenAPI/Swagger specification files defining API contracts, schemas, and endpoint documentation.
+## files
+## arch
+Declarative YAML/JSON specification files following the OpenAPI standard, organized as static schema definitions without executable code.
+## tags
+-
+## symbols
+-
+## workflows
+-
+## dirty
+-