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.

archive/.pi-map.index.md

@@ -0,0 +1,31 @@
+# archive (index)
+dir: archive
+
+## role
+Archive of an earlier project structure for a Streamlit-based Jellyfin media library browser with SSH remote file inspection capabilities.
+## parent
+index: ./.pi-map.index.md
+map: ./.pi-map.md
+## children
+- archive/src
+  index: archive/src/.pi-map.index.md
+  map: archive/src/.pi-map.md
+- archive/tests
+  index: archive/tests/.pi-map.index.md
+  map: archive/tests/.pi-map.md
+## files
+- app.py
+- pyproject.toml
+- requirements.txt
+## links
+index: archive/.pi-map.index.md
+map: archive/.pi-map.md
+## workflows
+- change archive behavior
+  read: app.py, pyproject.toml, requirements.txt
+- change archive config
+  read: pyproject.toml
+- explore archive subdirectories
+  index: archive/src/.pi-map.index.md, archive/tests/.pi-map.index.md
+## dirty
+-