Remove stale agent worktree and ignore .claude/worktrees going forward

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -29,6 +29,9 @@ data/study-sessions.json
 .env
 .env.*
 
+# Claude Code agent worktrees — local scratch space, never part of the repo
+.claude/worktrees/
+
 # Editor directories and files
 .vscode/*
 !.vscode/extensions.json