docs(kit): re-pin to core 1.21.0 and teach report targets

ci/core-ref.json moves from 7bd93b8 (1.20.0) to f13b8fe (1.21.0,
website#233, report, block and hide). Chapter 2 gains a section on
registerReportTargets, registerGameIdentities and ctx.safety, and its call
count becomes twenty-three. template/module.json's coreApi becomes ^1.21.0
for the equality check; the template declares none of the new members.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E14m6SuuY6i1vASFeGDBeY

template/module.json

@@ -2,7 +2,7 @@
   "id": "examplegame",
   "name": "Example Game",
   "version": "0.1.0",
-  "coreApi": "^1.20.0",
+  "coreApi": "^1.21.0",
   "server": "server/index.js",
   "client": { "entry": "client/dist/entry.js" },
   "schema": "server/db/schema.sql",