feat: add interface-*, cfe-* tests (batches 8+9) — all skills covered

- interface-edit, interface-validate: 2 cases
- cfe-init, cfe-validate, cfe-borrow, cfe-patch-method, cfe-diff: 5 cases
- runner: switch params in preRun (true = no value)
- Findings: interface-edit JSON-in-JSON, cfe-patch-method type naming

52 tests across 40 skills, all passing (~48s).

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

tests/skills/cases/cfe-diff/_skill.json

@@ -0,0 +1,12 @@
+{
+  "script": "cfe-diff/scripts/cfe-diff",
+  "setup": "empty-config",
+  "args": [
+    { "flag": "-ExtensionPath", "from": "workPath", "field": "extensionPath" },
+    { "flag": "-ConfigPath", "from": "workDir" }
+  ],
+  "snapshot": {
+    "root": "workDir",
+    "normalizeUuids": true
+  }
+}