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-instance-proxy/specs/.pi-map.md

@@ -1,13 +1,13 @@
-# openspec/changes/archive/2026-05-22-instance-proxy/specs
-dir: openspec/changes/archive/2026-05-22-instance-proxy/specs
+# archive/2026-05-22-instance-proxy/specs
+dir: archive/2026-05-22-instance-proxy/specs
 
-index: openspec/changes/archive/2026-05-22-instance-proxy/specs/.pi-map.index.md
+index: archive/2026-05-22-instance-proxy/specs/.pi-map.index.md
 
 ## role
-Contains archived OpenAPI specification files for an instance proxy feature from a specific dated change set.
+Contains specification documents defining requirements and interfaces for the instance proxy component.
 ## files
 ## arch
-Flat directory structure storing versioned JSON specification files as immutable snapshots of API definitions.
+Documentation-driven design with structured specification files establishing contracts for implementation.
 ## tags
 -
 ## symbols