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/_skill.json

@@ -0,0 +1,11 @@
+{
+  "script": "mxl-validate/scripts/mxl-validate",
+  "setup": "none",
+  "args": [
+    { "flag": "-TemplatePath", "from": "workPath", "field": "templatePath" }
+  ],
+  "snapshot": {
+    "root": "workDir",
+    "normalizeUuids": false
+  }
+}