feat(15-01): add markdownlint-cli2 + md:lint script + root config

- Install markdownlint-cli2@0.22.1 as root workspace devDependency
- Add md:lint script (no glob args — globs/ignores live in config file)
- Create .markdownlint-cli2.jsonc with prettier preset + content rules
- Enable MD001/MD024/MD040/MD031/MD051/MD052; disable MD041/MD034/MD036
- Glob: docs/**/*.md, *.md, apps/**/*.md; ignores .planning/** and node_modules

package.json

@@ -12,7 +12,8 @@
     "lint": "pnpm -r --if-present lint",
     "typecheck": "pnpm -r typecheck",
     "format": "prettier --write .",
-    "format:check": "prettier --check ."
+    "format:check": "prettier --check .",
+    "md:lint": "markdownlint-cli2"
   },
   "devDependencies": {
     "@eslint/js": "9.39.4",
@@ -20,6 +21,7 @@
     "eslint-config-prettier": "10.1.8",
     "eslint-plugin-react": "7.37.5",
     "eslint-plugin-react-hooks": "7.1.1",
+    "markdownlint-cli2": "0.22.1",
     "prettier": "3.8.4",
     "typescript-eslint": "8.61.0"
   }