Add approval-gated social publishing example

[kriptoburak]

Summary

• add a production-grade publish-social-update SKILL.md example for approval-gated social publishing
• include TweetClaw as an example of an external tool that still needs exact user approval and secret-safe handling
• repair the README feedback link now that the Discussions URL returns 404

Validation

• npm run lint
• npm test
• npm run build
• npx --yes markdown-link-check README.md spec/examples/publish-social-update/SKILL.md --quiet
• git diff --check

Note: npm ci completed with one pre-existing low-severity npm advisory in the target dependency tree; this PR does not change dependencies.