Add readability refinement prompt to ai-tools

[parthmax2]

Adds src/ai-tools/prompts/readability.md, a new prompt type for the ai-tools CLI that edits articles for clarity and scannability while preserving meaning, frontmatter, and Liquid syntax. Follows the full-file output style used by the versioning prompt, so it is auto-registered as --prompt readability with no code changes. This is one of the future refinement types listed in the ai-tools README.

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Closes:

What's being changed (if available, include any code snippets, screenshots, or gifs):

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• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

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[github-actions]

👋 Hey there spelunker. It looks like you've modified some files that we can't accept as contributions:

• src/ai-tools/prompts/readability.md

You'll need to revert all of the files you changed that match that list using GitHub Desktop or git checkout origin/main <file name>. Once you get those files reverted, we can continue with the review process.

The complete list of files we can't accept are:

• .devcontainer/**
• .github/**
• data/reusables/rai/**
• Dockerfile*
• src/**
• package*.json
• content/actions/how-tos/security-for-github-actions/security-hardening-your-deployments/**

We also can't accept contributions to files in the content directory with frontmatter contentType: rai.