feat: remove npm install/build and add keep_worktree option

- Remove npm install and npm run build from worktree initialization
- Remove INITIALIZING status enum and related code
- Add keep_worktree config option (default: false)
- When keep_worktree is true, worktree is preserved for debugging
- Renumber execution steps after removing initialize step

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

integration-tests/concurrent-runner/examples/toy-config.json

@@ -5,6 +5,7 @@
   "worktree_base": "~/.qwen/worktrees",
   "outputs_dir": "./examples/test-run",
   "results_file": "./examples/test-run/results.json",
+  "keep_worktree": false,
   "tasks": [
     {
       "id": "task-1",