docs: add the regen-baselines skill and command

[robsongajunior]

Summary

Adds the regen-baselines skill (.claude/skills/regen-baselines/SKILL.md) and its /regen-baselines <branch | pr> [modes] command (.claude/commands/regen-baselines.md), codifying the CI-only flow for refreshing the visual-regression baselines of a PR branch:

1. Trigger Storybook Regenerate Baseline (app-storybook-generate-baseline.yml) with update_baselines=true on the branch HEAD (optionally a modes subset).
2. Wait (~5 min warm), download the visual-baselines-linux artifact.
3. Mirror it into apps/storybook/.storybook/test-visual/__image_snapshots__/linux/ with rsync -a --delete.
4. Review the diff scope story-by-story against the branch's changes (unexplained stories stop the flow).
5. Commit only the snapshots folder (test(storybook): …) and push.

Includes a Conflict recovery section for when main and the branch regenerated the same PNGs (binary conflicts): finish the merge/rebase picking any side (or git rebase --skip a stale baseline-only commit), push, regenerate on the merged tree.

Notes

• Baselines are platform-specific: only CI (ubuntu) snapshots are valid — the skill forbids local generation on macOS.
• Frontmatter follows .claude/rules/authoring-docs.md (scope: webkit, enforced_by: [git-workflow, release-types, review]).
• Docs-only: no release effect.
• Flow proven on PRs feat(theme): add a translucent loading-placeholder token role #824 and fix(webkit): bring the calendar onto the shared field contract #827 (including fix(webkit): bring the calendar onto the shared field contract #827's rebase-conflict recovery).