chore(ci): the pinned core is on main now, not edge

The module system cut over on 2026-08-12 and website's `edge` branch was
deleted, so `ci/core-ref.json` named a branch that no longer exists.

The sha did not move. The pinned commit is an ancestor of `main`, the contract
is still MODULE_API_VERSION 1.5.0, and no chapter changed - this is a label
correction, not a re-pin, and deliberately not the moment d2 exists to create.

Nothing in CI reads the `branch` field: the workflow clones the repo and checks
out the sha, which is both why the cutover could not break the build and why a
wrong label here would have sat unnoticed indefinitely. The field is for the
person deciding whether a newer core is worth re-reading the book for, and a
branch that no longer exists tells them nothing. The `why` block now says which
half is load-bearing.

checkCoreApi.js's not-a-core error also asserted that core `main` "has none
until the cutover", which stopped being true at the same merge. A reader hitting
that message would have gone looking for a cutover that already happened - the
kit's own lesson from the CRLF defect, that a failure message naming a diagnosis
has to be right about it, applied to the kit's own scripts.

Verified both paths: the check still passes against a real core (^1.5.0 vs
1.5.0), and the rewritten message renders as intended. checkLinks,
checkRenameSites and checkChapterPaths all clean.

Co-Authored-By: Claude <noreply@anthropic.com>

scripts/checkCoreApi.js

@@ -43,8 +43,9 @@ const versionFile = path.resolve(corePath, 'server/src/modules/version.js')
 if (!fs.existsSync(versionFile)) {
   console.error(`checkCoreApi: ${versionFile} does not exist.`)
   console.error('  Either --core does not point at a website checkout, or the pin in')
-  console.error('  ci/core-ref.json names a ref with no module system in it (core `main`')
-  console.error('  has none until the cutover — see that file).')
+  console.error('  ci/core-ref.json names a core from before the module system existed')
+  console.error('  (it reached `main` at the 2026-08-12 cutover, so any ref older than')
+  console.error('  that on `main` has no server/src/modules/ at all).')
   process.exit(1)
 }