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>

ci/core-ref.json

@@ -1,6 +1,6 @@
 {
   "repo": "https://gitea.whitlocktech.com/RunicGateway/website.git",
-  "branch": "edge",
+  "branch": "main",
   "ref": "4ad8b2bb0ede2747622075dcfa4cb1fe460f91ca",
   "why": [
     "The core this kit is written against, pinned to a commit rather than a branch.",
@@ -15,10 +15,15 @@
     "MODULE_API_VERSION, so a contract bump turns this repo red on purpose",
     "(MODULE_SYSTEM.md 2.11.1 d2, 2.10).",
     "",
-    "The branch is `edge`, not `main`, and that is not a mistake: the module system",
-    "has not cut over yet and core's `main` has no server/src/modules/ at all",
-    "(MODULE_SYSTEM.md decision 11). This pin is one of the things that cutover has",
-    "to revisit.",
+    "The branch said `edge` until 2026-08-12, when the module system cut over",
+    "and that branch was deleted (MODULE_SYSTEM.md 2.9). The SHA DID NOT MOVE:",
+    "the pinned commit is an ancestor of `main`, so this is a label correction",
+    "and not a re-pin - the contract is still 1.5.0 and no chapter changed.",
+    "Nothing in CI reads this field; it clones the repo and checks out the sha,",
+    "which is why the cutover could not break the build and why a wrong label",
+    "here would have sat unnoticed. It 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.",
     "",
     "Same convention as Module-uo's ci/core-ref.json, deliberately - one file, one",
     "sha, reviewable in a diff."