chore: archive final 4 completed OpenSpec changes

Move the following completed changes from openspec/changes/ to
openspec/changes/archive/2026-06-12-completed-changes-archive/:
- multi-session-terminal-ux
- reorganize-long-files
- working-copies
- workspace-first-ui

Update parent and archive .pi-map*.md indexes to reflect the move and
remove the transient active-changes-archive grouping.

openspec/changes/ now contains only the archive/ directory.

openspec/changes/archive/2026-06-12-completed-changes-archive/workspace-first-ui/.pi-map.index.md

@@ -0,0 +1,23 @@
+# archive/2026-06-12-active-changes-archive/workspace-first-ui (index)
+dir: archive/2026-06-12-active-changes-archive/workspace-first-ui
+
+## role
+Archive of planning documents for a workspace-first UI redesign that reorients the product around persistent writable workspaces rather than direct repository browsing.
+## parent
+index: archive/2026-06-12-active-changes-archive/.pi-map.index.md
+map: archive/2026-06-12-active-changes-archive/.pi-map.md
+## children
+-
+## files
+- .openspec.yaml
+- design.md
+- proposal.md
+- spec.md
+- tasks.md
+## links
+index: archive/2026-06-12-active-changes-archive/workspace-first-ui/.pi-map.index.md
+map: archive/2026-06-12-active-changes-archive/workspace-first-ui/.pi-map.md
+## workflows
+-
+## dirty
+-