Show pull request descriptions in review view

[mikecfisher]

Summary

• collect GitHub PR and GitLab MR author/title/description metadata in review state
• render PR/MR source context above changed files with collapsible Markdown and safe embedded images
• pass truncated descriptions to walkthrough and review-assist prompts as author intent, and update the walkthrough guide

Demo

Validation

• vp check --fix
• vp build

[cpojer]

This is awesome, thank you!