feat: Replace cookiecutter with built-in MiniJinja templating

- Add MiniJinja dependency for Jinja2-compatible templating
- Create new project_template.rs module for template rendering
- Bundle snowflake template in templates/snowflake/ directory
- Remove dependency on external cookiecutter Python tool
- Handle Gitea publishing natively in Rust instead of Python hooks
- Update default template path to use bundled template with fallback
- Update README to reflect new built-in templating
- All tests pass

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

templates/snowflake/{{ cookiecutter.project_name }}/03-Characters/character_grid/character_grid.md

@@ -0,0 +1,20 @@
+<!-- A quick‑look table summarizing all characters (goal, motivation, stakes, etc.) -->
+
+<table>
+<thead>
+<tr>
+<th>Name</th>
+<th>Age</th>
+<th>Description</th>
+<th>Motivation</th>
+</tr>
+</thead>
+<tbody>
+<tr>
+<td>John Doe</td>
+<td>30</td>
+<td>Average Joe</td>
+<td>Just wants a cheeseburger.</td>
+</tr>
+</tbody>
+</table>