build: run doc-sync gates in the local pre-push hook too

The doc-sync gates were CI-only, so the AGENTS.md doc-sync promise could be
missed locally until after push. Add a shared `doc-sync` package.json script
(doc-typecheck + verify-event-taxonomy) wired into the lefthook pre-push job,
and point the CI step at the same script — one source of truth per ADR 0007.
Addresses PR review finding.

lefthook.yml

@@ -27,3 +27,6 @@ pre-push:
 
     - name: hygiene
       run: yarn hygiene
+
+    - name: doc-sync
+      run: yarn doc-sync