fix: Restrict CSpell to just markdown files at the configuration level

[easymac]

Not sure if this will work as intended.

This fix moves the file scoping into .cspell.yaml itself, the project-level config, so that no matter which runner invokes CSpell or how it's configured, only markdown files get checked. The Shorebird CI pulls in a separate.cspell.yaml workflow that runs CSpell with no file filter, so it checked all files. This should allow us to only be spell checking markdown as intended and not be failing CI (DEV) because of misspellings in, for example, package.json.

[eseidel]

Where was the file scoping before?

[easymac]

The GitHub Action .github/workflows/main.yaml was already passing includes: **/*.md, **/*.mdx, but the CI configuration pulls in a different configuration:

ci-worker-docs… workflowUrl: https://storage.googleapis.com/storage/v1/b/shorebird-workflows-dev/o/workflows%2F2007%2FCSpell.yaml?alt=media

Threw this at the wall to see if the project-level configuration would override it; seems to have failed. I'll have to look into CI more closely to see where/how configuration needs to be updated.

(CI would probably still fail on the codecov problem we fixed in prod anyway)

[eseidel]

The reason why I ask is that Shorebird CI is designed to be zero-config. So if there are other config spots that we aren't reading for CSpell and should be, that is a problem. But main.yaml is not a CSpell config spot, rather a GitHub Actions config so it seems fine for us to ignore. Thanks!