Bump markdownlint-cli2 to fix moderate security vulnerabilities

markdown-it ReDoS vulnerability (GHSA-38c4-r59v-3vqw) resolved
by upgrading markdownlint-cli2 from 0.19 to 0.21. Dev-only
dependency, no user impact.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -95,7 +95,7 @@
     "ignore": "^7.0.5",
     "jest": "^30.0.4",
     "lint-staged": "^16.1.1",
-    "markdownlint-cli2": "^0.19.1",
+    "markdownlint-cli2": "^0.21.0",
     "prettier": "^3.7.4",
     "prettier-plugin-packagejson": "^2.5.19",
     "semver": "^7.6.3",