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-validate/valid-template.json

@@ -0,0 +1,12 @@
+{
+  "name": "Корректный макет проходит валидацию",
+  "preRun": [
+    {
+      "script": "mxl-compile/scripts/mxl-compile",
+      "input": { "columns": 3, "areas": [{ "name": "Test", "rows": [["A", "B", "C"]] }] },
+      "args": { "-JsonPath": "{inputFile}", "-OutputPath": "Template.xml" },
+      "cwd": "{workDir}"
+    }
+  ],
+  "params": { "templatePath": "Template.xml" }
+}