Refuse a hand-typed npm publish

This repo's root is the published package, so 'npm publish' typed at the
repo root reaches the registry: it skips the tests, the changelog, the tag
and the GitHub release, and npm takes it whenever the working tree carries a
version the registry has not seen. Today a clean checkout carries the version
already published, so the registry answers 403 - a side effect, not a
decision, and it stops holding the moment somebody bumps by hand.

prepublishOnly now runs 'release-toolchain --assert-release', which passes
only while the release script is the one publishing. 'npm pack' runs prepack
instead, so reading a tarball still works and the pkg-pr-new previews, which
pack rather than publish, are untouched.

Also moves the toolchain pin to v1.2.0. The lockfile is the substantive half
of that: changing the spec alone left the previous SHA resolved, so 'npm ci'
kept installing v1.0.0 while package.json claimed otherwise.

Claude-Session: https://claude.ai/code/session_01XaYAhzmiJwysZeq1XC5xrQ

Author: Stefano Verna

package-lock.json

@@ -23,7 +23,8 @@
     "format": "biome check --write --unsafe",
     "changeset": "changeset",
     "release": "release-toolchain",
-    "release:next": "release-toolchain --tag next"
+    "release:next": "release-toolchain --tag next",
+    "prepublishOnly": "release-toolchain --assert-release"
   },
   "keywords": [
     "datocms",
@@ -55,7 +56,7 @@
   "devDependencies": {
     "@biomejs/biome": "^1.8.3",
     "@changesets/cli": "^3.0.1",
-    "@datocms/release-toolchain": "github:datocms/release-toolchain#v1.1.0",
+    "@datocms/release-toolchain": "github:datocms/release-toolchain#v1.2.0",
     "pkg-pr-new": "^0.0.88",
     "rimraf": "^5.0.7",
     "simple-git-hooks": "^2.11.1",