release: finalize v20.1.0 (community PRs + new skill) and harden check (#57)

- Roll the merged community PRs (#47 install safety, #48 prioritisation
  helper, #50 YouTube skill) and the now-174 skill count into the v20.1.0
  changelog and README 'What's New' / latest-release line.
- Harden 'npm run check' to rebuild web/skills.json and fail on drift, so a
  stale playground index can't pass locally and break CI (root cause of the
  check-generated failure after #48).


Claude-Session: https://claude.ai/code/session_016JWn5jRD5tcEFKrubjQ6Px

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

package.json

@@ -53,7 +53,7 @@
     "skillcheck": "node scripts/skillcheck.mjs",
     "build:exports": "node scripts/build-exports.mjs",
     "build:web": "node web/build-skills.mjs",
-    "check": "node scripts/skillcheck.mjs && node scripts/build-exports.mjs --check"
+    "check": "node scripts/skillcheck.mjs && node scripts/build-exports.mjs --check && node web/build-skills.mjs && git diff --exit-code -- web/skills.json"
   },
   "engines": {
     "node": ">=18"