chore: archive 15 completed OpenSpec changes

Move the following audited-and-implemented changes into
openspec/changes/archive/2026-06-12-completed-changes-archive/:

- backend-frontend-refactoring
- config-profile-git-mounts
- config-profile-includes-ui
- config-profile-multi-repo-mounts
- container-monitoring-notifications
- git-mount-url-validation
- home-path-expansion
- mobile-terminal-ux
- mount-specificity-ordering
- notification-center
- persistent-terminal-sessions
- session-list-overhaul
- ssh-key-mounting
- terminal-fullscreen-unified-header
- tool-session-progress-and-updates

Also regenerated .pi-map*.md files for openspec/changes so the
remaining active changes (multi-session-terminal-ux, reorganize-long-files,
working-copies, workspace-first-ui) reflect the new layout.

openspec/changes/archive/2026-06-12-completed-changes-archive/.pi-map.md

@@ -0,0 +1,19 @@
+# archive/2026-06-12-completed-changes-archive
+dir: archive/2026-06-12-completed-changes-archive
+
+index: archive/2026-06-12-completed-changes-archive/.pi-map.index.md
+
+## role
+Stores historical records of fully implemented and audited OpenSpec changes for reference and audit trail purposes.
+## files
+- README.md | Documents archived completed OpenSpec changes that have been audited and confirmed as fully implemented
+## arch
+Simple flat-file archive using date-based directory naming and markdown documentation for immutable change tracking.
+## tags
+readme, documents, archived, completed, openspec, changes, have, been
+## symbols
+-
+## workflows
+-
+## dirty
+-