Add social source review skill example

[kriptoburak]

Summary

• Add examples/social-source-review, a compact Agent Skill package for approval-gated X/Twitter source review with TweetClaw as the concrete source tool.
• Document the example and cover it with a CLI exit-code regression test.

Validation

• go run ./cmd/skill-validator check examples/social-source-review
• go test -race ./... -count=1
• Markdown links checked with markdown-link-check

[kriptoburak]

Optional visibility note: if this is useful and you merge it after checks pass, feel free to share the PR on X/Twitter and tag https://x.com/burakbayir. I can repost it to my 24k followers so more developers and power users can discover this repo.