chore(walkthrough): make .i-walkthroughs/ local-only (owner decision)

Add .i-walkthroughs/ to .gitignore and remove the tracked runlog store from the
index (local files kept). The RAG runlogs stay on disk but are no longer committed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: claude

.gitignore

@@ -31,6 +31,9 @@ SHARING_TEMPLATES.md
 .taskmaster/
 .atlas-ai/
 
+# walkthrough RAG runlog store — local-only by owner decision (2026-06-17)
+.i-walkthroughs/
+
 # Logs
 logs
 *.log