docs: add TweetClaw external skill example

[kriptoburak]

Summary

• Add TweetClaw as a concrete dormant external skill example for public X/Twitter persona workflows.
• Document skillssh:xquik-dev/tweetclaw and the OpenClaw plugin install command.
• Keep credentials in the host runtime or plugin config and require confirmation for visible X/Twitter actions.

Validation

• npm ci passes; npm reports 1 existing high severity advisory without dependency changes.
• node scripts/validate-skills.mjs passes.
• git diff --check passes.
• Added-line secret-pattern scan passes.
• Added-line dash scan passes.
• npx --yes node@20 --test tests/refine.test.js fails on the existing applyRefinement compliance-gate assertion even with this docs change stashed, so it appears pre-existing and unrelated to this patch.
• npm test fails for the same pre-existing assertion after 840 of 841 tests pass.