fix: use python3 for correctness checks and add CI (#50)

The benchmark harness hardcoded `python`, which is missing on macOS and
many Linux images. Probe python3 first, add npm test, and run checks in
GitHub Actions so regressions are caught on every PR.

Co-authored-by: Cursor <cursoragent@cursor.com>

package.json

@@ -4,6 +4,9 @@
   "description": "Lazy senior dev mode for AI agents. The best code is the code you never wrote.",
   "keywords": ["pi-package", "pi", "skills", "ponytail"],
   "license": "MIT",
+  "scripts": {
+    "test": "node --test tests/*.test.js && npm test --prefix pi-extension"
+  },
   "pi": {
     "extensions": ["./pi-extension/index.js"],
     "skills": ["./skills"]