docs: scaffold the Integration Kit — front page, outline, and the checks

Phase 5 slice 0 (MODULE_SYSTEM.md §2.11.1). The repo's governance, the front
page, the book's outline, and the CI that keeps the whole thing from rotting.

README.md
  What the reader is building, all three parts, and the draft banner: the kit is
  finished when someone outside this project builds a working module by
  following it alone, and that has not happened. Says the sidecar rule plainly
  (MODULE_API.md §2.7) rather than leaving it to chapter 3, because a reader who
  skims the front page and starts coding should still get that one right.

book/README.md
  The outline of four chapters, landed before the prose so the shape can be
  argued with. Chapters are named but NOT linked — a link to a file that does
  not exist is what the link check is for, and an outline should not be the
  first thing to fail it.

CONTRIBUTING.md
  The rule that governs every change here: the kit never re-specifies a
  contract. Also the prose conventions, and why the pinned ref points at core's
  `edge` rather than `main`.

SECURITY.md
  Scoped for a repo that runs nothing: the two things that ARE reportable are a
  template that teaches an insecure pattern (it is meant to be copied) and a
  chapter that teaches something dangerous.

scripts/checkLinks.js
  Relative links resolve; anchors match a real heading; no link pins a reader to
  a commit snapshot of a moving document. Nothing is fetched — a self-hosted
  Gitea would fail on a credential-less runner and teach us to ignore red.
  Fences and code spans are stripped by a line walk, not a regexp.

  Its first run found a real one: a PR template's relative links resolve from
  the REPO ROOT, because that is where their text ends up when Gitea inlines
  them into a pull request body. Encoded, with the reason.

scripts/checkCoreApi.js
  The anti-rot check. Asserts template/module.json's `coreApi` EQUALS the pinned
  core's MODULE_API_VERSION — equality, not "satisfies", because a range check
  stays green across a contract bump and green would then mean "the template
  still loads" instead of "someone has re-read the book". Both failure branches
  and the pass were exercised against a real core checkout.

ci/core-ref.json
  The pin, same convention as Module-uo's. Points at `edge`: core's `main` has
  no server/src/modules/ until the cutover, and that pin is one of the things
  the cutover has to revisit.

.gitea/workflows/pr-checks.yml
  Two jobs. `links` always runs; `template` is conditional on
  template/module.json existing, so the repo is gated now and the job arms
  itself when slice 1 lands, with no edit to the workflow. Same guard Module-uo
  used through its planning phase.

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

ci/core-ref.json

@@ -0,0 +1,20 @@
+{
+  "repo": "https://gitea.whitlocktech.com/RunicGateway/website.git",
+  "branch": "edge",
+  "ref": "c3120ea3daee579ea9948e9e466065f25ee4e92f",
+  "why": [
+    "The core this kit is written against, pinned to a commit rather than a branch.",
+    "Moving this pin is the moment someone re-reads the chapters: CI asserts the",
+    "version template/module.json declares still equals this core's",
+    "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.",
+    "",
+    "Same convention as Module-uo's ci/core-ref.json, deliberately - one file, one",
+    "sha, reviewable in a diff."
+  ]
+}