fix: gitignore dotfolders, ensure .devcontainer/ created for services

- Added .ruff_cache/, .agents/, .tmp_*/ to .gitignore
- setup_project.py: mkdir .devcontainer/ before writing docker-compose.yml
  (directory no longer ships with template after devcontainer extraction)

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

Author: stranma

setup_project.py

@@ -425,6 +425,7 @@ def configure_devcontainer_services(root: Path, services: str, replacements: dic
     """
     actions = []
     devcontainer_dir = root / ".devcontainer"
+    devcontainer_dir.mkdir(exist_ok=True)
 
     # Write docker-compose.yml from template
     template = COMPOSE_TEMPLATES[services]