Add delete repository feature

GitService.deleteRepository() removes both worktree and git-dir.
Exposed via POST /repo/{name}/delete, a 'Danger Zone' section on
the repo page, and telnet main menu option 4 with confirmation
prompt. Includes unit tests for all layers.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/main/resources/templates/repo.html

@@ -99,5 +99,12 @@ New branch: <input type="text" name="branch" size="20">
 </tr>
 </table>
 
+<hr>
+
+<h3>Danger Zone</h3>
+<form method="post" th:action="@{/repo/{name}/delete(name=${name})}">
+<input type="submit" value="Delete Repository">
+</form>
+
 </body>
 </html>