Preserve developer-written PR descriptions

[simple-analytics-ai]

Summary

Fixes simpleanalytics/dashboard#856

• Classify description provenance from the PR author, edit history, body markers, commit attribution, and agent branch signals, while giving direct human authorship priority.
• Preserve a developer-written description word for word except for the minimum normalization required by Summary, Security implications, Testing, and Checklist.
• Restore every pre-review image or attachment if an automated rewrite drops it, including Markdown, HTML, reference-style images, and GitHub user attachments.
• Limit automation-created checklist items to Linked to an issue, Tested, and Asked for a review; preserve additional items only when a developer already wrote them.
• Protect a newer human edit made while the review is running and document the behavior in the shared-workflow README.

Security implications

Has security impact - described as: The workflow continues to use pull-request write permissions, but generated description changes are now constrained by author provenance, protected-media restoration, and a fixed workflow checklist.

Testing

• actionlint .github/workflows/pull-request.yml
• Ruby YAML safe-load of .github/workflows/pull-request.yml
• Focused execution of the embedded provenance and finalizer scripts covering human precedence, bot authorship, concurrent human edits, screenshot restoration, reference-style images, and checklist limits
• git diff --check

Checklist

• Linked to an issue
• Tested
• Asked for a review