feat: add mxl-* tests (batch 3), support cwd in skill config and preRun

- mxl-compile, mxl-validate, mxl-info, mxl-decompile: 4 cases
- runner: cwd option in _skill.json and preRun steps for skills
  that resolve OutputPath relative to current directory
- Finding: mxl-compile only accepts relative OutputPath

21 tests across 9 skills, all passing.

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

tests/skills/cases/mxl-compile/simple-template.json

@@ -0,0 +1,18 @@
+{
+  "name": "Простой макет с одной областью",
+  "input": {
+    "columns": 3,
+    "areas": [
+      {
+        "name": "Заголовок",
+        "rows": [
+          ["Наименование", "Количество", "Сумма"]
+        ]
+      }
+    ]
+  },
+  "params": { "outputPath": "Template.xml" },
+  "expect": {
+    "files": ["Template.xml"]
+  }
+}