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/skills/.pi-map.md

@@ -0,0 +1,18 @@
+# .opencode/skills
+dir: .opencode/skills
+
+index: .opencode/skills/.pi-map.index.md
+
+## role
+Directory for defining custom agent skills, capabilities, and behavioral instructions within the opencode configuration framework.
+## files
+## arch
+Configuration-based skill definition directory; skills are declared as individual files consumed by the opencode agent runtime to extend or specialize assistant behavior.
+## tags
+-
+## symbols
+-
+## workflows
+-
+## dirty
+-