fix(cli): add allow_unicode=True and encoding="utf-8" to YAML I/O (#1936)

None of the yaml.dump() calls specify allow_unicode=True, causing
non-ASCII characters in extension descriptions to be escaped to
\uXXXX sequences in generated .agent.md frontmatter and config files.

Add allow_unicode=True to all 6 yaml.dump() call sites, and
encoding="utf-8" to all corresponding write_text() and read_text()
calls to ensure consistent UTF-8 handling across platforms.

tests/test_extensions.py

@@ -747,6 +747,18 @@ $ARGUMENTS
         assert output.endswith("---\n")
         assert "description: Test command" in output
 
+    def test_render_frontmatter_unicode(self):
+        """Test rendering frontmatter preserves non-ASCII characters."""
+        frontmatter = {
+            "description": "Prüfe Konformität der Implementierung"
+        }
+
+        registrar = CommandRegistrar()
+        output = registrar.render_frontmatter(frontmatter)
+
+        assert "Prüfe Konformität" in output
+        assert "\\u" not in output
+
     def test_register_commands_for_claude(self, extension_dir, project_dir):
         """Test registering commands for Claude agent."""
         # Create .claude directory