Remove GitHub agent markdown file from cspell exclusion

This issue is to tack a follow up work item to remove sdk generation agent md from cspell exclusion list.

I think we run cspell on most MD files by default, so we should be able to on this one as well?

_Originally posted by @mikeharder in https://github.com/Azure/azure-rest-api-specs/pull/40547#discussion_r2829909911_

Run cspell on agent workflow markdown file.

Locate the SDK generation agent Markdown workflow and the cspell exclusion entry referenced in the issue. Remove that exclusion, run cspell against the workflow Markdown file, and confirm the check passes without spelling errors.

Written by the indexing model from the issue text.