docs: add OpenClaw package skill guidance

[kriptoburak]

Summary

• add integration guidance for OpenClaw plugin packages that ship openclaw.plugin.json and packaged skills
• use the live @xquik/tweetclaw package layout as a concrete example, while keeping Skillflag CLI wiring explicitly opt-in
• add the required independent X Corp disclosure
• fix 2 pre-existing Skillflag spec link-check blockers: the stale Goose skills URL and the frontmatter email autolink

Checks

• rebased onto current upstream main
• verified current @xquik/tweetclaw package and plugin metadata from the canonical repository and npm registry
• npm ci
• npm run format:check
• npm run lint
• npm run build
• npm test (54 passed)
• npx --yes markdown-link-check@3.13.7 docs/SKILLFLAG_SPEC.md -q
• npx --yes markdown-link-check@3.13.7 docs/INTEGRATION.md -q
• git diff --check

Published as 1 SSH-signed commit: 8bcf3d4c35ffb2c132794126d89aceaa20d3d234.

[kriptoburak]

Repair published on current upstream as 1 GitHub-verified signed commit. I also checked the example against the live TweetClaw package layout, made future Skillflag CLI wiring explicit, added the required independent-service disclosure, and retained the 2 unrelated broken-spec-link fixes. All repository checks pass, including 54 tests and both Markdown link audits.