fix(engines): lower Node.js minimum to 22

Node 22 is still in Active LTS until October 2026 and Maintenance LTS
until April 2027. Raising the engines floor to >=24.0.0 unnecessarily
locked out a fully-supported LTS version and produced EBADENGINE
warnings on install. Restore Node 22 support, add Node 22 to the CI
matrix, and update CONTRIBUTING.md to match.

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

package.json

@@ -36,7 +36,7 @@
     "url": "https://github.com/gsd-build/get-shit-done/issues"
   },
   "engines": {
-    "node": ">=24.0.0"
+    "node": ">=22.0.0"
   },
   "devDependencies": {
     "c8": "^11.0.0",