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/workspace-first-ui/.pi-map.index.md

@@ -1,11 +1,11 @@
-# openspec/changes/workspace-first-ui (index)
-dir: openspec/changes/workspace-first-ui
+# workspace-first-ui (index)
+dir: workspace-first-ui
 
 ## role
-Design and planning documents for a workspace-first UI refresh that restructures the application around persistent writable workspaces rather than direct repository browsing.
+Design and planning documentation for a workspace-first UI refresh that restructures the application around persistent writable workspaces rather than direct repository browsing.
 ## parent
-index: openspec/changes/.pi-map.index.md
-map: openspec/changes/.pi-map.md
+index: ./.pi-map.index.md
+map: ./.pi-map.md
 ## children
 -
 ## files
@@ -14,8 +14,8 @@ map: openspec/changes/.pi-map.md
 - spec.md
 - tasks.md
 ## links
-index: openspec/changes/workspace-first-ui/.pi-map.index.md
-map: openspec/changes/workspace-first-ui/.pi-map.md
+index: workspace-first-ui/.pi-map.index.md
+map: workspace-first-ui/.pi-map.md
 ## workflows
 -
 ## dirty