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/2026-05-22-session-management-fixes/specs/.pi-map.md

@@ -1,13 +1,13 @@
-# openspec/changes/archive/2026-05-22-session-management-fixes/specs
-dir: openspec/changes/archive/2026-05-22-session-management-fixes/specs
+# archive/2026-05-22-session-management-fixes/specs
+dir: archive/2026-05-22-session-management-fixes/specs
 
-index: openspec/changes/archive/2026-05-22-session-management-fixes/specs/.pi-map.index.md
+index: archive/2026-05-22-session-management-fixes/specs/.pi-map.index.md
 
 ## role
-Contains archived specification documents for session management fixes from a May 2026 release, serving as historical documentation for past API design decisions.
+Contains archived technical specifications for session management fixes from a specific dated release.
 ## files
 ## arch
-Flat directory structure storing dated specification snapshots with no active code or architectural patterns—purely document archival using filesystem-based versioning.
+Document-based specification storage with date-versioned archiving pattern for tracking historical design decisions.
 ## tags
 -
 ## symbols