Add TweetClaw to Agent Skills

[kriptoburak]

Summary

• Add a Social Media agent skills category with TweetClaw.
• Link to the maintained upstream TweetClaw skill, GitHub repo, and npm package.
• Adjust three existing descriptions so the awesome-lint no-repeat item check is clean.

Checks

• npx --yes markdownlint-cli2 README.md skills/social/tweetclaw/README.md
• npx --yes awesome-lint README.md still reports remark-lint:awesome-github locally because this temporary clone is not recognized as the canonical GitHub repository.
• npx --yes markdownlint-cli2 "**/*.md" "#node_modules" still reports pre-existing table-style issues in the existing Azure skill files.