chore(worktree): auto-run yarn install on new worktrees

Add a Claude Code SessionStart hook (matcher: startup) that runs
`corepack yarn install` when node_modules is missing, so worktrees Claude
creates internally are immediately usable. Also extend create-task-worktree.sh
to install deps after the worktree is created, covering Codex/Cursor/manual
flows that share that script.

scripts/create-task-worktree.sh

@@ -60,6 +60,12 @@ echo "Creating branch $branch_name from $base_ref"
 echo "Creating worktree at $worktree_path"
 git worktree add "$worktree_path" -b "$branch_name" "$base_ref"
 
+if [ -f "$worktree_path/yarn.lock" ]; then
+  echo ""
+  echo "Installing dependencies in new worktree (corepack yarn install)..."
+  (cd "$worktree_path" && corepack yarn install)
+fi
+
 echo ""
 echo "Worktree ready."
 echo "Branch: $branch_name"