chore: drop plan/ from git, ignore locally and purge on deploy

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

.gitignore

@@ -16,5 +16,4 @@ application/database.php
 sync_config.jsonc
 __MACOSX/
 *.code-workspace
-plan/chat-backup/**/*.jsonl
-plan/chat-backup/subagents/
+/plan/