Parse MkDocs nav with YAML

> [!CAUTION]
> Protected files were modified in this change.
> This pull request is in `request_review` mode and requires explicit human scrutiny before merge.
>
> Protected files: `pyproject.toml`, `uv.lock`

## Summary

- Parse `mkdocs.yml` nav entries with `yaml.safe_load()` and walk the parsed nav tree before collecting workflow slugs.
- Keep direct `python3 scripts/check-nav-catalog.py` compatible when PyYAML is unavailable by stripping YAML line comments in the fallback path.
- Add regression coverage for inline YAML comments containing workflow paths.

Closes \#1460.

## Testing

- `python3 scripts/check-nav-catalog.py`
- `UV_CACHE_DIR=/tmp/gh-aw/agent/uv-cache UV_LINK_MODE=copy make test`

---
[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Framework Best Practices](https://github.com/elastic/ai-github-actions/actions/runs/28448713032)

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---

> [!NOTE]
> This was originally intended as a pull request, but the git push operation failed.
>
> **Workflow Run:** [View run details and download bundle artifact](https://github.com/elastic/ai-github-actions/actions/runs/28448713032)
>
> The bundle file is available in the `agent` artifact in the workflow run linked above.

To create a pull request with the changes:

```sh
# Download the artifact from the workflow run
gh run download 28448713032 -n agent -D /tmp/agent-28448713032

# Fetch the bundle into a temporary ref, then update the local branch
git fetch /tmp/agent-28448713032/aw-fix-nav-catalog-yaml-parsing.bundle refs/heads/fix-nav-catalog-yaml-parsing:refs/bundles/create-pr-fix-nav-catalog-yaml-parsing-77992e4ff6b64390-6fb12244
git update-ref refs/heads/fix-nav-catalog-yaml-parsing-77992e4ff6b64390 refs/bundles/create-pr-fix-nav-catalog-yaml-parsing-77992e4ff6b64390-6fb12244
git checkout fix-nav-catalog-yaml-parsing-77992e4ff6b64390
# Ensure the working tree matches the updated branch
git reset --hard
# Remove the temporary bundle ref
git update-ref -d refs/bundles/create-pr-fix-nav-catalog-yaml-parsing-77992e4ff6b64390-6fb12244

# Push the branch to origin
git push origin fix-nav-catalog-yaml-parsing-77992e4ff6b64390

# Create the pull request
gh pr create --title 'Parse MkDocs nav with YAML' --base main --head fix-nav-catalog-yaml-parsing-77992e4ff6b64390 --repo elastic/ai-github-actions
```

Start with scripts/check-nav-catalog.py and inspect how it currently reads mkdocs.yml and collects workflow slugs. Run python3 scripts/check-nav-catalog.py and the listed make test command; done means the parsed nav handles inline YAML comments, the PyYAML-unavailable path remains compatible, and regression coverage passes.

Written by the indexing model from the issue text.