docs: clarify bookmark review safety boundaries

[kriptoburak]

Summary

• add a first-run safety checklist for the bookmark folder, artifact directory, message destination, and Processed-folder move behavior
• clarify that bookmark-brain has no hosted backend or analytics, while configured message channels and linked articles can still receive data
• document TweetClaw only as an optional separate evidence companion for adjacent X/Twitter jobs, not as part of bookmark-brain
• fix pre-existing root-relative README example links so link checkers resolve them inside this repository

Validation

• git diff --check
• npx --yes markdown-link-check README.md SKILL.md examples/example-analysis.md --quiet
• curl -fsSI https://github.com/Xquik-dev/tweetclaw

[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 or mention burakbayir. I can repost it to my 24k followers so more developers and power users can discover this repo.