chore: remove chat-backup JSONL from repo, keep markdown summaries only

Co-authored-by: Cursor <cursoragent@cursor.com>

.gitignore

@@ -15,4 +15,6 @@ node_modules
 application/database.php
 sync_config.jsonc
 __MACOSX/
-*.code-workspace
+*.code-workspace
+plan/chat-backup/**/*.jsonl
+plan/chat-backup/subagents/