Improvements: two-column layout, better commit messages, preview URL in PRs, and diff library

[fedacking]

Summary

Feature improvement suggestions from user feedback:

1. Two-column layout — Keep the conversation always visible on one side, with a switchable panel (logs, diff, preview, info) on the other. Similar to the Stripe Minions UI pattern.
2. Better commit messages — Currently all commits within a task have the same generic message. Each commit should have a contextual message describing what actually changed in that step.
3. Auto-add Preview URL to PR body — When a preview exists and a PR is created from Tekton, the preview URL should be included in the PR body even if automated previews are not configured.
4. Evaluate @pierre/diffs library — Consider using diffs.com (@pierre/diffs) as an open source diff rendering library to improve the diff viewing experience.

Source

User feedback from Damian.