chore(gitignore): ignore Memory Bank session checkpoints

The PreCompact hook writes checkpoints to .memory-bank/session/. Like
promptHistory.md they are local session state and must not be committed.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: AI Assistant <ai@example.com>

.gitignore

@@ -244,5 +244,6 @@ ModelManifest.xml
 # FAKE - F# Make
 .fake/
 
-# Memory Bank: local prompt history, not version-controlled
+# Memory Bank: local prompt history and session checkpoints, not version-controlled
 /.memory-bank/promptHistory.md
+/.memory-bank/session/