docs: restore markdownlint section in CONTRIBUTING.md

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: tarun-etikala

CONTRIBUTING.md

@@ -47,7 +47,18 @@ ruff format --check . # format check
 ruff format .         # auto-format
 ```
 
-Configuration is in [`ruff.toml`](ruff.toml) at the repo root. For markdownlint config, see [`.markdownlint.jsonc`](.markdownlint.jsonc) (rules) and [`.markdownlint-cli2.yaml`](.markdownlint-cli2.yaml) (ignored paths).
+Configuration is in [`ruff.toml`](ruff.toml) at the repo root.
+
+### Markdown (markdownlint)
+
+Run locally before pushing:
+
+```bash
+npx markdownlint-cli2@0.22.1 "**/*.md"        # lint
+npx markdownlint-cli2@0.22.1 --fix "**/*.md"  # auto-fix
+```
+
+Configuration is in [`.markdownlint.jsonc`](.markdownlint.jsonc) (rules) and [`.markdownlint-cli2.yaml`](.markdownlint-cli2.yaml) (ignored paths) at the repo root.
 
 ## Commit message conventions