[codex] Add Markdown Prettier CI check (#3242)

## Summary

- Add a Markdown Prettier check to the existing Check Markdown Links
workflow
- Skip symlinked Markdown paths so Prettier does not fail on explicit
symlink inputs
- Format `.claude/commands/stress-test.md` so the new gate passes on
current `main`

## Why

Markdown-only PRs currently skip the `Lint JS and Ruby` workflow, so
`prettier --check .` does not run until a later code PR triggers it.
This lets unformatted Markdown land on `main` and then breaks unrelated
PRs.

## Test Plan

- `actionlint .github/workflows/check-markdown-links.yml`
- Markdown Prettier script from the workflow
- pre-commit hook
- pre-push hook over SSH

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk: workflow-only changes plus markdown reformatting; main risk
is CI becoming stricter and failing on existing markdown formatting or
pnpm/prettier setup issues.
> 
> **Overview**
> Adds a **Prettier `--check` gate for Markdown** to the existing
`check-markdown-links` GitHub Actions workflow, including pnpm/Node
setup and skipping symlinked markdown files.
> 
> Expands the workflow trigger paths to include formatting-related
config/lockfiles (`.prettierrc`, `.prettierignore`, `pnpm-lock.yaml`)
and reformats `.claude/commands/stress-test.md` (tables/emphasis) so it
passes the new CI check.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
930a4b8. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: justin808