feat(skills): add pr-design-doc skill

[smolpaws]

• A human has tested these changes.

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Why

Reviewers of a non-trivial PR often have to reconstruct the design from the diff alone — the shape of the change, the API before/after, and why this approach. That's slow and error-prone. This skill guides an agent to hand maintainers a single, self-contained HTML design doc they can grasp in ~15 seconds, grounded to real code.

It also operationalizes the .pr/ convention: the doc is committed under the temporary .pr/ directory (auto-removed on approval by pr-artifacts.yml), and linked in the PR description via an htmlpreview URL that renders before the PR is merged.

Companion guideline issue: OpenHands/OpenHands#16304.

Summary

• Add the pr-design-doc skill (/pr-design-doc, /design-doc): when to use it (and when to skip — trivial PRs), the .pr/ workflow, a step-by-step process, what the page contains (decision-first summary, API before/after, per-file before/after cards grounded to path:line), and anti-patterns.
• Include references/html-craft.md — the editorial look, hand-drawn before/after SVG technique, code-grounding discipline, and the htmlpreview delivery link (adapted so .pr/ + the fork/branch URL is the primary path).
• Register in the openhands-extensions marketplace + README catalog; add the .plugin manifest, vendor symlinks, and generated command files via scripts/sync_extensions.py.

Adapted from the show-me visualization skill, focused on the OpenHands PR-review workflow.

Issue Number

Related: OpenHands/OpenHands#16304

How to Test

# registry/catalog/symlinks/commands are all in sync
uv run python scripts/sync_extensions.py --check   # only the pre-existing, unrelated issue-duplicate-checker coverage warning

# skill structure + plugin-loading + sync tests
uv run pytest tests/test_skills_have_readme.py tests/test_skill_plugin_loading.py tests/test_sync_extensions.py tests/test_catalogs.py -q   # 117 passed

You can also read skills/pr-design-doc/SKILL.md and follow it against any non-trivial PR: it produces .pr/design.html and an htmlpreview link of the shape
https://htmlpreview.github.io/?https://github.com/<fork-owner>/<repo>/blob/<pr-branch>/.pr/design.html.

Notes

• Skills-only change; no integration catalog or code touched.
• The .pr/ auto-removal workflow lives in OpenHands/OpenHands; this skill documents relying on it.

Co-authored-by: smolpaws engel@enyst.org

[all-hands-bot]

👋 This PR needs a couple of things fixed before OpenHands can review it:

• the PR description's HUMAN: section needs at least 20 characters describing what you tested, not just the template placeholder

Push an update once this is addressed and this check re-runs automatically.

This is an automated check - no AI was used to generate this comment.