refactor: remove jinja2 dependency, use simple template engine

- Replace jinja2 with custom render_template() helper
- Support {{ var }} substitution and {% if %}...{% endif %} conditionals
- Remove jinja2 from pyproject.toml and requirements.txt
- Simplifies dependency tree and reduces install size

pyproject.toml

@@ -27,7 +27,6 @@ classifiers = [
 dependencies = [
     "anthropic>=0.75.0",
     "google-genai>=0.1.0",
-    "jinja2>=3.1.6",
     "openai>=2.11.0",
     "pyyaml>=6.0.3",
     "questionary>=2.1.1",