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.

.opencode/.pi-map.md

@@ -0,0 +1,18 @@
+# .opencode
+dir: .opencode
+
+index: .opencode/.pi-map.index.md
+
+## role
+Configuration directory for the opencode tool, managing project-specific settings and preferences.
+## files
+## arch
+Flat directory structure containing configuration files that define opencode behavior for the associated project.
+## tags
+-
+## symbols
+-
+## workflows
+-
+## dirty
+-