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/unify-tasks-on-services/.pi-map.index.md

@@ -0,0 +1,21 @@
+# openspec/changes/archive/unify-tasks-on-services (index)
+dir: openspec/changes/archive/unify-tasks-on-services
+
+## role
+Archived OpenSpec change proposal that documented the migration of saved task execution from machine-based to SSH service-based references.
+## parent
+index: openspec/changes/archive/.pi-map.index.md
+map: openspec/changes/archive/.pi-map.md
+## children
+-
+## files
+- design.md
+- proposal.md
+- tasks.md
+## links
+index: openspec/changes/archive/unify-tasks-on-services/.pi-map.index.md
+map: openspec/changes/archive/unify-tasks-on-services/.pi-map.md
+## workflows
+-
+## dirty
+-