fix(ci): add npm ci + build:lib before version sync in auto-backmerge

The auto-backmerge workflow runs sync-next-version.cjs which calls npm version,
triggering the version lifecycle hook (gen-capability-registry.cjs). That script
requires the compiled capability-validator.cjs (a build:lib artifact), which was
never built on the backmerge runner — causing the backmerge to fail with
'capability validation failed — registry not written'. Add the build step.

.github/workflows/auto-backmerge.yml

@@ -141,6 +141,11 @@ jobs:
             echo "dropped_oneline=" >> "$GITHUB_OUTPUT"
           fi
 
+
+      - name: Install dependencies and build
+        if: steps.check.outputs.next_exists == 'true'
+        run: npm ci --silent && npm run build:lib
+
       - name: Sync next's version to main's released version
         if: steps.check.outputs.next_exists == 'true'
         run: |