docs: add OpenClaw package skill guidance

[kriptoburak]

Summary

• add integration guidance for OpenClaw plugin packages that already ship openclaw.plugin.json and skills/<id>/SKILL.md
• use @xquik/tweetclaw as a concrete package-shape example for keeping OpenClaw install metadata native while exposing portable Skillflag exports
• fix two pre-existing Skillflag spec link-check blockers: the stale Goose skills URL and the frontmatter email autolink

Checks

• duplicate check: no target PRs/issues/content found for TweetClaw, Xquik, @xquik/tweetclaw, OpenClaw, ClawHub, or x-twitter-scraper before editing
• target policy: MIT license, non-fork upstream, personal fork parent verified
• npm ci
• npx prettier --check docs/INTEGRATION.md docs/SKILLFLAG_SPEC.md
• npx --yes markdown-link-check docs/INTEGRATION.md docs/SKILLFLAG_SPEC.md --quiet
• npm run lint
• npm test
• git diff --check