chore: update CI matrix to Node 20, 22, 24 — drop EOL Node 18

Node 18 reached EOL April 2025. Node 24 is the current LTS target.

Changes:
- CI matrix: [18, 20, 22] → [20, 22, 24]
- package.json engines: >=16.7.0 → >=20.0.0
- Removed Node 18 conditional in CI (c8 coverage works on all 20+)
- Simplified CI to single test:coverage step for all versions

797/797 tests pass on Node 24.

package.json

@@ -36,7 +36,7 @@
     "url": "https://github.com/glittercowboy/get-shit-done/issues"
   },
   "engines": {
-    "node": ">=16.7.0"
+    "node": ">=20.0.0"
   },
   "devDependencies": {
     "c8": "^11.0.0",