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.index.md

@@ -0,0 +1,23 @@
+# .opencode (index)
+dir: .opencode
+
+## role
+Configuration directory for the opencode tool, managing project-specific settings and preferences.
+## parent
+index: ./.pi-map.index.md
+map: ./.pi-map.md
+## children
+- .opencode/commands
+  index: .opencode/commands/.pi-map.index.md
+  map: .opencode/commands/.pi-map.md
+- .opencode/skills
+  index: .opencode/skills/.pi-map.index.md
+  map: .opencode/skills/.pi-map.md
+## files
+## links
+index: .opencode/.pi-map.index.md
+map: .opencode/.pi-map.md
+## workflows
+-
+## dirty
+-