fix(docs): scope fenced code language linting

[pimlock]

Summary

Configure Markdown linting so bare fenced code blocks are allowed in internal docs while published docs under docs/ still require a language.

This keeps architecture and planning documents free to use bare fences for diagrams and terminal sketches, while preserving stricter rendering/copy behavior for user-facing Fern docs.

Example

Bare fence (no type). This is now allowed in internal docs.

```
Something
```

Fence with a type.

```text
Something
```

Changes

• Disable MD040 in the root markdownlint config.
• Re-enable MD040 in docs/.markdownlint-cli2.jsonc.

Testing

• mise run markdown:lint:md passes
• Verified a synthetic bare fence under docs/ still fails MD040
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

[github-actions]

🌿 Preview your docs: https://nvidia-preview-pr-965.docs.buildwithfern.com/openshell