chore: ignore in-repository git worktrees

.worktrees/ showed as untracked in main's status, so a worktree checkout could
be committed into the repository by an ordinary 'git add -A'.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -25,3 +25,7 @@ artifacts/tests/visual/
 .env
 .env.*
 !.env.example
+
+# Git worktrees created inside the repository. A worktree is a checkout, not
+# source: committing one would nest a second working copy inside this one.
+.worktrees/