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/.pi-map.md

@@ -0,0 +1,18 @@
+# openspec/changes/archive
+dir: openspec/changes/archive
+
+index: openspec/changes/archive/.pi-map.index.md
+
+## role
+Provides historical archive storage for tracked changes, enabling audit trails and rollback capabilities for OpenSpec documents.
+## files
+## arch
+Append-only immutable storage pattern with timestamp-versioned snapshots, likely using file-based or object-store backend with content-addressable indexing for deduplication.
+## tags
+-
+## symbols
+-
+## workflows
+-
+## dirty
+-