chore: track .pi-map.md and .pi-map.index.md artifacts

Remove the map files from .gitignore so they are committed as project
navigation artifacts, and also unignore them in the sample fixture.
Regenerate all maps so the committed versions reflect the current source.

fixtures/.pi-map.index.md

@@ -0,0 +1,20 @@
+# fixtures (index)
+dir: fixtures
+
+## role
+Provides test data and setup utilities for automated testing across the project.
+## parent
+index: ./.pi-map.index.md
+map: ./.pi-map.md
+## children
+- fixtures/sample-project
+  index: fixtures/sample-project/.pi-map.index.md
+  map: fixtures/sample-project/.pi-map.md
+## files
+## links
+index: fixtures/.pi-map.index.md
+map: fixtures/.pi-map.md
+## workflows
+-
+## dirty
+-