fix: guard cd in formula_worktree_setup with || return (SC2164)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/formula-session.sh

@@ -310,7 +310,7 @@ build_sdk_prompt_footer() {
 # Requires globals: PROJECT_REPO_ROOT, PRIMARY_BRANCH.
 formula_worktree_setup() {
   local worktree="$1"
-  cd "$PROJECT_REPO_ROOT"
+  cd "$PROJECT_REPO_ROOT" || return
   git fetch origin "$PRIMARY_BRANCH" 2>/dev/null || true
   worktree_cleanup "$worktree"
   git worktree add "$worktree" "origin/${PRIMARY_BRANCH}" --detach 2>/dev/null