fix(ci): drop Node 22 from matrix, require Node 24 minimum (#1848)

Node 20 reached EOL April 30 2026. Node 22 is no longer the LTS
baseline — Node 24 is the current Active LTS. Update CI matrix to
run only Node 24, raise engines floor to >=24.0.0, and update
CONTRIBUTING.md node compatibility table accordingly.

Fixes #1847

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

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