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/changes/archive/2026-06-17-web-ui-rework/specs/.pi-map.md

@@ -0,0 +1,18 @@
+# openspec/changes/archive/2026-06-17-web-ui-rework/specs
+dir: openspec/changes/archive/2026-06-17-web-ui-rework/specs
+
+index: openspec/changes/archive/2026-06-17-web-ui-rework/specs/.pi-map.index.md
+
+## role
+Specifications directory for the web UI rework change proposal in the OpenSpec archive.
+## files
+## arch
+Empty directory intended to contain structured specification documents following the OpenSpec change-driven workflow pattern.
+## tags
+-
+## symbols
+-
+## workflows
+-
+## dirty
+-