chore: track all project map and index files

Add generated .pi-map.md and .pi-map.index.md files across the repository
so the project navigation maps are shared and versioned. These artifacts
are maintained by project_map_init/patch/validate and must be kept in
sync with source edits.

Note: .cache/ remains ignored (added in previous commit).

openspec/changes/archive/2026-05-25-mobile-pages-overhaul/specs/.pi-map.md

@@ -0,0 +1,18 @@
+# openspec/changes/archive/2026-05-25-mobile-pages-overhaul/specs
+dir: openspec/changes/archive/2026-05-25-mobile-pages-overhaul/specs
+
+index: openspec/changes/archive/2026-05-25-mobile-pages-overhaul/specs/.pi-map.index.md
+
+## role
+Contains archived specification documents for a mobile pages overhaul project from May 2026, serving as historical design reference for the openspec changes tracking system.
+## files
+## arch
+Simple archival storage pattern with date-based directory hierarchy (YYYY-MM-DD-prefixed) for immutable specification snapshots, using flat file organization within versioned folders.
+## tags
+-
+## symbols
+-
+## workflows
+-
+## dirty
+-