Add proofread_markdown skill and Dart wrapper script

[antfitch]

Proofread markdown skill for the Flutter Docs repo. You can use this skill to update your markdown files to follow the Flutter doc teams formatting guidelines.

• Changes title case to sentence case in headings.
• Converts headings from passive case to active case (note: when working with how-to and tutorials, this is a good thing, but there are times when this should be avoided, such as when you're introducing concepts. Look carefully at the changes before accepting them.
• Ensures there is a blank line between heading and body text.
• Refactors steps for brevity.

This is an early version of this skill. As time goes on, we'll update it to be more complete, fix bugs, etc. Use this with your AI agents, but in a mode where you can approve the changes.

[gemini-code-assist]

Code Review

This pull request introduces a new markdown proofreading skill and a supporting Dart script for automated line wrapping. The review feedback identifies a hardcoded absolute path in the documentation that needs to be corrected to a relative path. Additionally, the line-wrapping script has limitations in detecting ordered lists beyond three items and contains logic errors in the wrapText function that cause indentation loss and inaccurate width calculations.

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 6f3d0fb):

https://flutter-docs-prod--pr13258-add-proofreading-skill-k2seal0i.web.app

[sfshaza2]

See comments.

[antfitch]

Add the format that @parlough is using here to separate sections: https://github.com/flutter/website-cms/blob/main/.agents/skills/generate-showcase-from-video/SKILL.md

[parlough]

Add the format that @parlough is using here to separate sections: https://github.com/flutter/website-cms/blob/main/.agents/skills/generate-showcase-from-video/SKILL.md

Just clarifying: That new, fancy skill is by @abdallahshaban557 :)

[sfshaza2]

I can live with this! :D. lgtm

[sfshaza2]

Oops! Forgot to click approve.

[antfitch]

I can live with this! :D. lgtm

YAY! I had forgotten for a moment that of course we could have a semantic line break script. Just took a few days. @conooi you could update the dart skill with this, too. Should save lots of tokens.